Closes https://github.com/cesanta/mos-libs/pull/2

CL: Add MG_HIDE_SERVER_INFO

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CL: Fix TZ on ESP8266

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CL: none

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mgos_dlsym needs to include mg_str for ffi_exports to be able to use it

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Enables extra compile-time checks

CL: none

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Also remove `-O3` optimization flasg, it negates `-Os`. Saves ~400 bytes.

CL: none

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CL: Add `mg_url_encode_opt()` - a parametrized version of `mg_url_encode()`

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Per standard, cookies are delimited by `; `.

CL: Fix mg_http_parse_header: treat ";" as a delimiter.

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CL: Apache license to common/

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A major cleanup, disentangling net_if and ssl_if.
Pulled a lot of common logic into the core and reduced size of net_if implementations.

CL: Mongoose net_if and ssl_if refactoring

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Consume buffer as soon as we know there is no boundary there, no need to delay until next chunk arrives.
This prevents stall where buffer fills up in one go and next chunk never arrives.

CL: Fix an edge case in multipart HTTP upload parsing

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In preparation for making moving to lib.
Reduce amount of makefile copypasta a bit.

CL: none

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 * Limit total amount of headroom, in absolute terms (`MBUF_SIZE_MAX_HEADROOM`).
 * If unable to allocate with headroom, fall back to allocating the required minimum.
 * For mOS, set default `MBUF_SIZE_MULTIPLIER` to 2 to avoid floating point operations.
   Since max headroom size is now capped to 128 bytes, this will not result in much of a bloat.

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mongoose: fix formatting

mongoose: fix formatting

mongoose: remove debugging messages

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freshen: implement ota rollback

freshen: api for rpc

freshen: implement rpc api (without transport)

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CL: Fix mg_socketpair for lwip + UDP

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Fixes https://github.com/cesanta/mongoose-os/issues/394

CL: Fix buffer size passed to inet_ntop()

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E.g. if `ASSERT_EQ(myfunc(), 123)` fails, `myfunc()` would be invoked again to print the actual value.
This can cause confusion sometimes.
To avoid this, we cast arguments to double, which is wide enough for most cases and we provide a variant for 64 bit value types for when it's not.
We also perform a check and fail if argument to the check results in loss of precision.

CL: none

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CL: none

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Which is a replacement of (deprecated) `mg_http_parse_header`, but,
similarly to `asprintf`, allocates a new buffer if the client-provided
one is not large enough.

Also use it throughout mongoose code, and thus some header-related
limitations are removed; in particular,
https://github.com/cesanta/mongoose/issues/813 is fixed.

CL: Mongoose Web Server: Deprecate `mg_http_parse_header()` and implement `mg_http_parse_header2()` instead, which allocates a new buffer if the client-provided one is not large enough (similarly to `asprintf`).
CL: Mongoose Web Server: Fix limitations of header value lengths, e.g. when parsing authentication headers such as nonce, etc.

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CL: none

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To properly fail a test in a subroutine, return value needs to be passed up.
This snippet makes it short.

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CL: Mongoose Web Server: Add UDP client example

Resolves https://github.com/cesanta/mongoose/issues/886

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In certain files it was necessary to make tests work from public
mongoose repo, so this commit makes things consistent.

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Even though it makes no prectical difference due to the way amalgamation works

CL: none

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CL: none

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CL: Mongoose Web Server: Remove tunneling functionality which was used by nobody

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CL: Mongoose Web Server: Publish sources and tests

Resolves https://github.com/cesanta/mongoose/issues/745

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Before it resulted in an endless recursion.

Resolves https://github.com/cesanta/mongoose/issues/871

CL: Mongoose Web Server: Fail if passed a NULL handler to `mg_bind` or `mg_bind_opt`.

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CL: Mongoose Web Server: Digest authentication: Fix nonce validity check (expired nonce or nonce from the future did not cause the the check to fail)
CL: Mongoose Web Server: Digest authentication: Fix nonce request value; it worked before because nonce validity check was broken as well
CL: Mongoose Web Server: Digest authentication: Add `nonce` argument to `mg_http_create_digest_auth_header()`: clients should use the value received from the server's authentication request.

Resolves https://github.com/cesanta/mongoose/issues/809

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