Migrate what used to be in lws_hdr_related into either the ah
or the wsi, and eliminate it along with the three different ways
used to access things inside it.
Eg wsi->u.hdr.ah and wsi->u.http.ah become wsi->ah
These changes are internal-only, in private-libwebsockets.h and lib.
This enables selected things from -Wextra, can't use -Wextra because it is
fussy enough to complain about unused params on functions... they are
there for a reason.
-Wsign-compare
-Wignored-qualifiers
not -Wimplicit-fallthrough=3 ... only on gcc 7
-Wtype-limits
-Wuninitialized
not -Wclobbered ... only on gcc 7ish
fix the warnings everywhere they were found.