Kernel/Userland: Add a halt syscall, and a shutdown binary to invoke it

Author: https://github.com/rburchell
Commit: 952382b413
Pull-request: https://github.com/SerenityOS/serenity/pull/232
Reviewed-by: https://github.com/awesomekling

Kernel/Syscall.h

@@ -112,7 +112,8 @@ struct timeval;
     __ENUMERATE_SYSCALL(getpeername)            \
     __ENUMERATE_SYSCALL(sched_setparam)         \
     __ENUMERATE_SYSCALL(sched_getparam)         \
-    __ENUMERATE_SYSCALL(fchown)
+    __ENUMERATE_SYSCALL(fchown)                 \
+    __ENUMERATE_SYSCALL(halt)
 
 namespace Syscall {