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
Make restful_server and netcat examples compilation tests:
- netcat doesn't use HTTP, compile it with MG_DISABLE_HTTP
- restful_server doesn't use websocket, compile it with
MG_DISABLE_HTTP_WEBSOCKET
h/t @labourcier, cesanta/mongoose#665
PUBLISHED_FROM=4494ac3cbd81e8314beb557ab301b4b44c4afb89
