FileSystem: Port most of the code over to using custodies.

The current working directory is now stored as a custody. Likewise for a
process executable file. This unbreaks /proc/PID/fd which has not been
working since we made the filesystem bigger.

This still needs a bunch of work, for instance when renaming or removing
a file somewhere, we have to update the relevant custody links.

Kernel/FileSystem/Custody.h

@@ -23,6 +23,8 @@ public:
 
     const String& name() const { return m_name; }
 
+    String absolute_path() const;
+
 private:
     Custody(Custody* parent, const String& name, Inode&);