Split parts of bits/socket.h to bits/sockettypes.h

[Roland McGrath]

> Can we start documenting this stuff piece-by-piece under "Internals
> Documentation" in the wiki?
> 
> We have a "Porting" section in the manual which could also do with
> documentation.

We might want to consider choosing one place or the other for that sort of
documentation.  It might make sense to stop having the manual talk about
developing libc (as opposed to using it or building/installing it), and
keep all that sort of content on the wiki.


Thanks,
Roland