initdb needs pqsignal() even on Windows.

I had thought we weren't using this version of pqsignal() at all on
Windows, but that's wrong --- initdb is using it (and coping with the
POSIX-ish semantics of bare signal() :-().  So allow the file to be
built in WIN32+FRONTEND case, and add it to the MSVC build logic.
Re-include pqsignal() in libpq.

We need this in non-ENABLE_THREAD_SAFETY builds, and also to satisfy
the exports.txt entry; while it might be a good idea to remove the
latter, I'm hesitant to do so except in the context of an intentional
ABI break.  At least we don't have a separately maintained source file
for it anymore.