Add some basic signal support.

It only works for sending a signal to a process that's in userspace code.

We implement reception by synthesizing a PUSHA+PUSHF in the receiving process
(operating on values in the TSS.)
The TSS CS:EIP is then rerouted to the signal handler and a tiny return
trampoline is constructed in a dedicated region in the receiving process.

Also hacked up /bin/kill to be able to send arbitrary signals (kill -N PID)

Userland/cat.cpp

@@ -1,7 +1,9 @@
-#include <LibC/stdio.h>
-#include <LibC/unistd.h>
-#include <LibC/errno.h>
-#include <LibC/string.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <stdlib.h>
+#include <assert.h>
 
 int main(int argc, char** argv)
 {