Get rid of not-very-portable fcntl(F_SETLK) mechanism for locking the Unix

Get rid of not-very-portable fcntl(F_SETLK) mechanism for locking the Unix
socket file, in favor of having an ordinary lockfile beside the socket file.
Clean up a few robustness problems in the lockfile code.  If postmaster is
going to reject a connection request based on database state, it will now
tell you so before authentication exchange not after.  (Of course, a failure
after is still possible if conditions change meanwhile, but this makes life
easier for a yet-to-be-written pg_ping utility.)