Kernel: PID/PGID typing

This compiles, and fixes two bugs:
- setpgid() confusion (see previous commit)
- tcsetpgrp() now allows to set a non-empty process group even if
  the group leader has already died. This makes Serenity slightly
  more POSIX-compatible.

Kernel/Syscalls/kill.cpp

@@ -52,7 +52,8 @@ KResult Process::do_killpg(pid_t pgrp, int signal)
     // Send the signal to all processes in the given group.
     if (pgrp == 0) {
         // Send the signal to our own pgrp.
-        pgrp = pgid();
+        // FIXME: PIF/PGID INCOMPLETE
+        pgrp = pgid().value();
     }
 
     bool group_was_empty = true;