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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CL: Mongoose Web Server: Websocket: Respond to Ping with Pong
CL: Mongoose Web Server: Websocket: Properly close a connection with Close frame (in response to a client's close and when protocol failure is detected)
CL: Mongoose Web Server: Websocket: Fix support of fragmented messages
CL: Mongoose Web Server: Websocket: Add support for control frames interjected in the middle of a fragmented message

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`path_info` was dereferenced without checking for NULL, and a few lines
below, it was checked for NULL.

CL: none

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CL: none

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CL: none

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Integrates https://github.com/cesanta/mongoose/pull/884
Closes https://github.com/cesanta/mongoose/issues/883

CL: Mongoose Web Server: Don't use user_data in mqtt server

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CL: Mongoose Web Server: Fix mg_parse_multipart in case of malformed request

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CL: none

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CL: Mongoose Web Server: Fix publish-subscribe example

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CL: Mongoose Web Server: Fix socket leak when there are too many open file descriptors

Fixes https://github.com/cesanta/mongoose/issues/870

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Fixes https://github.com/cesanta/mjs/issues/82
Integrates https://github.com/cesanta/mjs/pull/85

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CL: Fix OOB access in mg_match_prefix_n().
    Made mg_match_prefix_n() return non-negative result.

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CL: Fix TCP RX path regression in SSL mode

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CL: - Mongoose Web Server: close undrained and stuck http connections

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h/t @kzyapkov

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AWS MQTT connection works

Also fixes https://github.com/mongoose-os-libs/ethernet/issues/1 (because why not).

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Standardize common packages in build images, to make better use of Docker image cache.

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To avoid name clashes.

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2.0 redefined LWIP_VERSION in a way that is no longer compatible with use by preprocessor (boo).
also, tcp_pcb.acked is gone, but we only use it for debug, so it doesn't matter.

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