libpq-fe.h: Don't claim SOCKTYPE in the global namespace

libpq-fe.h: Don't claim SOCKTYPE in the global namespace

The definition of PGoauthBearerRequest uses a temporary SOCKTYPE macro
to hide the difference between Windows and Berkeley socket handles,
since we don't surface pgsocket in our public API. This macro doesn't
need to escape the header, because implementers will choose the correct
socket type based on their platform, so I #undef'd it immediately after
use.

I didn't namespace that helper, though, so if anyone else needs a
SOCKTYPE macro, libpq-fe.h will now unhelpfully get rid of it. This
doesn't seem too far-fetched, given its proximity to existing POSIX
macro names.

Add a PQ_ prefix to avoid collisions, update and improve the surrounding
documentation, and backpatch.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAOYmi%2BmrGg%2Bn_X2MOLgeWcj3v_M00gR8uz_D7mM8z%3DdX1JYVbg%40mail.gmail.com
Backpatch-through: 18