`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.
PUBLISHED_FROM=f8b66d5772846126b94223a729a6bd87897f8de6
On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.
PUBLISHED_FROM=5519526cf84e2bbd425a726fcc112fea1a95cbf1
