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)

Kernel/Syscall.h

@@ -56,6 +56,7 @@ enum Function {
     Getdtablesize = 0x2024,
     Dup = 0x2025,
     Dup2 = 0x2026,
+    Sigaction = 0x2027,
 };
 
 void initialize();