LibCore+LibSystem: Move syscall wrappers from LibSystem to LibCore

With this change, System::foo() becomes Core::System::foo().

Since LibCore builds on other systems than SerenityOS, we now have to
make sure that wrappers work with just a standard C library underneath.

Userland/Libraries/LibCore/System.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <signal.h>
+
+namespace Core::System {
+
+#ifdef __serenity__
+ErrorOr<void> pledge(StringView promises, StringView execpromises);
+ErrorOr<void> unveil(StringView path, StringView permissions);
+#endif
+
+ErrorOr<void> sigaction(int signal, struct sigaction const* action, struct sigaction* old_action);
+
+}