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

Userland/shutdown.cpp

@@ -0,0 +1,17 @@
+#include <Kernel/Syscall.h>
+#include <LibCore/CArgsParser.h>
+
+int main(int argc, char** argv)
+{
+    CArgsParser args_parser("shutdown");
+    args_parser.add_arg("n", "shut down now");
+    CArgsParserResult args = args_parser.parse(argc, (const char**)argv);
+
+    if (args.is_present("n")) {
+        syscall(SC_halt);
+        return 0;
+    } else {
+        args_parser.print_usage();
+        return 0;
+    }
+}