This worked fine for superusers, but not for ordinary users trying to
cancel their own processes.  Tweak the order the checks are done in so
that we correctly return SIGNAL_BACKEND_ERROR (which current callers
know to ignore without erroring out) so that an ordinary user can loop
through a resultset without fearing that a process might exit in the
middle of said looping -- causing the remaining processes to go
unsignalled.

Incidentally, the last in-core caller of IsBackendPid() is now gone.
However, the function is exported and must remain in place, because
there are plenty of callers in external modules.

Author: Josh Kupershmidt

Reviewed by Noah Misch