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`MG_ENABLE_...` macros are not yet renamed because it involves some
logic change: e.g.  instead of defining `MG_ENABLE_MQTT`, one should
define `MIOT_ENABLE_MQTT`, which should imply `MG_ENABLE_MQTT` as well.

Macro refactoring is going to be done in another PR.

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Also, do not require -DRTOS_SDK and pick socket/low-level event manager
based on LWIP_SOCKET.

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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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"#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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On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

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Explicitly loosen them on the "big" platforms.

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