pg_signal_backend() asymmetry

Hi all,

I have one nitpick related to the recent changes for
pg_cancel_backend() and pg_terminate_backend(). If you use these
functions as an unprivileged user, and try to signal a nonexistent
PID, you get:

ERROR: must be superuser or have the same role to cancel queries
running in other server processes

Whereas if you do the same thing as a superuser, you get:

WARNING: PID 123 is not a PostgreSQL server process
pg_cancel_backend
-------------------
f
(1 row)

The comment above the WARNING generated for the latter case says:

/*
* This is just a warning so a loop-through-resultset
will not abort
* if one backend terminated on its own during the run
*/

which is nice, but wouldn't unprivileged users want the same behavior?
Not to mention, the ERROR above is misleading, as it claims the
nonexistent PID really belongs to another user.

A simple fix is attached. The existing code called both
BackendPidGetProc(pid) and IsBackendPid(pid) while checking a
non-superuser's permissions, which really means two separate calls to
BackendPidGetProc(pid). I simplified that to down to a single
BackendPidGetProc(pid) call.

Josh