Further refactoring of c.h and nearby files.

Further refactoring of c.h and nearby files.

This continues the work of commit 91aec93e6 by getting rid of a lot of
Windows-specific funny business in "section 0".  Instead of including
pg_config_os.h in different places depending on platform, let's
standardize on putting it before the system headers, and in consequence
reduce win32.h to just what has to appear before the system headers or
the body of c.h (the latter category seems to include only PGDLLIMPORT
and PGDLLEXPORT).  The rest of what was in win32.h is moved to a new
sub-include of port.h, win32_port.h.  Some of what was in port.h seems
to better belong there too.

It's possible that I missed some declaration ordering dependency that
needs to be preserved, but hopefully the buildfarm will find that
out in short order.

Unlike the previous commit, no back-patch, since this is just cleanup
not a prerequisite for a bug fix.

Discussion: https://postgr.es/m/29650.1510761080@sss.pgh.pa.us