Kernel: Support signaling all processes with pid == -1

This is a special case that was previously not implemented.
The idea is that you can dispatch a signal to all other processes
the calling process has access to.

There was some minor refactoring to make the self signal logic
into a function so it could easily be easily re-used from do_killall.

Kernel/Process.h

@@ -445,6 +445,8 @@ private:
 
     KResult do_kill(Process&, int signal);
     KResult do_killpg(pid_t pgrp, int signal);
+    KResult do_killall(int signal);
+    KResult do_killself(int signal);
 
     KResultOr<siginfo_t> do_waitid(idtype_t idtype, int id, int options);