LibCore: Add LockFile, a filesystem based mutex

This API wraps flock(2) and also handles the file creation and deletion
when the LockFile goes out of scope.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -15,6 +15,7 @@ set(SOURCES
     IODevice.cpp
     LocalServer.cpp
     LocalSocket.cpp
+    LockFile.cpp
     MimeData.cpp
     NetworkJob.cpp
     NetworkResponse.cpp