Add proc-pgroup flog category

I'm not *super*-happy with this, because pgroups and terminal
ownership and such are quite entertwined.

But hey, if all fails just use `proc'*'`

src/postfork.cpp

@@ -124,7 +124,7 @@ bool set_child_group(job_t *j, pid_t child_pid) {
                 // ever leads to a terminal hang due if both this setpgid call AND posix_spawn's
                 // internal setpgid calls failed), write to the debug log so a future developer
                 // doesn't go crazy trying to track this down.
-                debug(2, "Error %d while calling setpgid for child %d (probably harmless)", errno,
+                FLOGF(proc_pgroup, "Error %d while calling setpgid for child %d (probably harmless)", errno,
                       child_pid);
             }
         }