Remove dllist.c from libpq. It's overkill for what libpq needs; we can

Remove dllist.c from libpq.  It's overkill for what libpq needs; we can
just stick a list-link into struct PGnotify instead.  Result is a smaller
faster and more robust library (mainly because we reduce the number of
malloc's and free's involved in notify processing), plus less pollution
of application link-symbol namespace.