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.cpp

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/String.h>
+#include <LibCore/Process.h>
+#include <errno.h>
+#include <spawn.h>
+#include <unistd.h>
+
+#ifdef __serenity__
+#    include <serenity.h>
+#endif
+
+namespace Core {
+
+pid_t Process::spawn(StringView path)
+{
+    String path_string = path;
+
+    pid_t pid;
+    char const* argv[] = { path_string.characters(), nullptr };
+    if ((errno = posix_spawn(&pid, path_string.characters(), nullptr, nullptr, const_cast<char**>(argv), environ))) {
+        perror("Process::spawn posix_spawn");
+    } else {
+#ifdef __serenity__
+        if (disown(pid) < 0)
+            perror("Process::spawn disown");
+#endif
+    }
+    return pid;
+}
+
+}