Kernel+LibC: Implement fcntl(2) advisory locks

Advisory locks don't actually prevent other processes from writing to
the file, but they do prevent other processes looking to acquire and
advisory lock on the file.

This implementation currently only adds non-blocking locks, which are
all I need for now.

Kernel/FileSystem/FileDescription.h

@@ -127,6 +127,9 @@ public:
 
     FileBlockCondition& block_condition();
 
+    KResult apply_flock(Process const&, Userspace<flock const*>);
+    KResult get_flock(Userspace<flock*>) const;
+
 private:
     friend class VirtualFileSystem;
     explicit FileDescription(File&);