libwebsockets.h: split out into a dir of sub-includes included by libwebsockets.h

This has no effect on user code or backward compatibility.

It moves the in-tree public api header libwebsockets.h from ./lib
to ./include, and introduces a dir ./include/libwebsockets/

The single public api header is split out into 31 sub-headers
in ./include/libwebsockets.  ./include/libwebsockets.h contains
some core types and platform adaptation code, but the rest of it
is now 31 #include <libwebsockets/...>

At install time, /usr/[local/]include/libwebsockets.h is installed
as before, along now with the 31 sub-headers in ...include/libwebsockets/

There's no net effect on user code.

But the api header is now much easier to maintain and study, with 31
topic-based sub headers.