Kernel: Userspace signal handlers are now called one at a time

I added a syscall for telling the kernel when signal execution has
finished. We should send a random hash or id to the signal trampoline
that we would include in the syscall, so validity of signal exit can
be confirmed.

libc/include/sys/syscall.h

@@ -44,6 +44,7 @@ __BEGIN_DECLS
 #define SYS_RAISE 37
 #define SYS_KILL 38
 #define SYS_SIGNAL 39
+#define SYS_SIGNAL_DONE 40
 
 __END_DECLS