LibCore: Add Core::Process::spawn()

This is a simple wrapper around posix_spawn() that will help us simplify
a bunch of very verbose posix_spawn() invocations.

This first version only supports the simplest case: executing an
executable without passing arguments or doing anything fancy. More
features can be added to cover more cases. :^)

Userland/Libraries/LibCore/Process.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace Core {
+
+class Process {
+public:
+    static pid_t spawn(StringView path);
+};
+
+}