Beautify bits of the hotcopying code.

* subversion/libsvn_fs_fs/fs.c
  (uninitialize_fs_struct): New utility function.
  (fs_hotcopy): Use that new function to make the init/uninit symmetry
                more obvious and add more commentary.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1616338 13f79535-47bb-0310-9956-ffa450edef68