Update compiler version to 5.2.8

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LWIP part has been finally fully split from the ESP part, and ESP
platform defs reduced to bare minimum. It is now possible to use LWIP
in either low-level or socket mode without any ESP dependencies,
and NXP Kinetis platform is the first to make use of it.

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Updated with changes.

Disable sync resolver by default.

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Possible values:
  MG_NET_IF_SOCKET (1) - traditional BSD socket API
  MG_NET_IF_SIMPLELINK (2) - TI's SimpleLink socket interface
  MG_NET_IF_LWIP_LOW_LEVEL - a net_if implementation that uses LWIP's
    low-level API, when LWIP_SOCKET is not available.

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mbed-os is a directory that gets checked out by the mbed cli
(a tool similar in spirit to miot) and it contains a lot of
.c sources, so many of them that clang-format burns your CPU.

While we're at it, also fix check_no_ifdefs when running under a commit
hook.

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Factor out SSI and WS code from http.c

HTTP + WS are enabled by default, but MQTT is built without HTTP
support as a compilation test.

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The build has been broken because MQTT has been disabled during some refactoring
but never caught because this example was never built on CI.

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Also factor out CGI-related code from http.c, it's getting too big.

Build tests with -Wundef

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"#if FOO" still works with simple -DFOO, but gives more flexibility.
Specifically, if user expressed no preference (FOO is not defined),
we can apply reasonable defaults (this is the legitimate use of ifdef).

In short, from now on, please use

 #if MG_ENABLE_FOO

instead of

 #ifdef MG_ENABLE_FOO

Since we are all used to #ifdef, this change also adds a precommit check
to police this. Specifically, in *.h and *.c files that are Copyright Cesanta,
"ifdef" and "if defined()" are not allowed to be used with macros that contain
ENABLE or DISABLE, unless the like also contains "ifdef-ok".

Hence, if you are sure you want to use ifdef, use this:

 #ifdef MG_ENABLE_FOO /* ifdef-ok */

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esp_mg_net_if was a rat's nest of mongoose net_if and event manager
implementaions and the event loop task for the ESP8266 miot port.
From that, pieces that are related to LWIP support have been pulled out,
separated into net_if and event manager files, and event loop task
has been moved under the miot esp8266 dirrectory.

This is done to facilitate LWIP code reuse. This may not be the end of
it, but it's a start.

Note: custom retransmit logic has been removed for now. Can be
reintroduced later is necessary.

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Currently this allows V7 to be built on MBED; more
work is likely required to port mongoose.

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This is the variant of MG_EV_SSI_CALL with context of the SSI call being
processed in addition to the tag argument. Specifically, call handler
now gets access to the HTTP request and the name of the file being processed.

MG_EV_SSI_CALL is preserved for backward compatibility.

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