Use pqsignal() in contrib programs rather than calling signal(2) directly.

The semantics of signal(2) are more variable than one could wish; in
particular, on strict-POSIX platforms the signal handler will be reset
to SIG_DFL when the signal is delivered.  This demonstrably breaks
pg_test_fsync's use of SIGALRM.  The other changes I made are not
absolutely necessary today, because the called handlers all exit the
program anyway.  But it seems like a good general practice to use
pqsignal() exclusively in Postgres code, now that we have it available
everywhere.