Use abort() instead of exit() to abort library functions

Use abort() instead of exit() to abort library functions

In some hopeless situations, certain library functions in libpq and
libpgport quit the program.  Use abort() for that instead of exit(),
so we don't interfere with the normal exit codes the program might
use, we clearly signal the abnormal termination, and the caller has a
chance of catching the termination.

This was originally pointed out by Debian's Lintian program.