SystemServer: Read service list from a config file

This replaces the hardcoded services list with a very simple config
file in /etc/SystemServer.ini :^)

Closes https://github.com/SerenityOS/serenity/issues/610

Servers/SystemServer/Service.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <AK/String.h>
+#include <LibCore/CObject.h>
+
+class CConfigFile;
+
+namespace AK {
+class JsonObject;
+}
+
+class Service final : public CObject {
+    C_OBJECT(Service)
+
+public:
+    void spawn();
+
+    void save_to(AK::JsonObject&) override;
+
+private:
+    Service(const CConfigFile&, const StringView& name);
+
+    // Path to the executable. By default this is /bin/{m_name}.
+    String m_executable_path;
+    // Extra arguments, starting from argv[1], to pass when exec'ing.
+    Vector<String> m_extra_arguments;
+    // File path to open as stdio fds.
+    String m_stdio_file_path;
+    int m_priority { 1 };
+    // The name of the user we should run this service as.
+    String m_user;
+    uid_t m_uid { 0 };
+    gid_t m_gid { 0 };
+
+    // PID of the running instance of this service.
+    pid_t m_pid { -1 };
+
+    void resolve_user();
+};