Fix pg_dump's sigTermHandler() to use _exit() not exit().

Fix pg_dump's sigTermHandler() to use _exit() not exit().

sigTermHandler() tried to be careful to invoke only operations that
are safe to do in a signal handler.  But for some reason we forgot
that exit(3) is not among those, because it calls atexit handlers
that might do various random things.  (pg_dump itself installs no
atexit handlers, but e.g. OpenSSL does.)  That led to crashes or
lockups when attempting to terminate a parallel dump or restore
via a signal.

Fix by calling _exit() instead.

Per bug #16199 from Raúl Marín.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/16199-cb2f121146a96f9b@postgresql.org