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.
PUBLISHED_FROM=925ed9a55abb193ed7deac1c9675a0e5b35dca50
"#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 */
PUBLISHED_FROM=9be829448f53cff575d6cae8b9945fb12531c15a
