Allow libpq to do thread-safe SIGPIPE handling. This allows it to

Allow libpq to do thread-safe SIGPIPE handling.  This allows it to
ignore SIGPIPE from send() in libpq, but terminate on any other SIGPIPE,
unless the user installs their own signal handler.

This is a minor fix because the only time you get SIGPIPE from libpq's
send() is when the backend dies.