Refactor VIMUSERFILES concept:

- VIMUSERFILES now points directly to the individual user's customized
  directory (e.g., ~/.vim/user/someuser), instead of to that directory's
  parent.  This gives better naming, as $VIMUSERFILES is analogous to
  the $VIMFILES directory.

- The ~/.vimuser directory no longer requires a $VIMUSER subdirectory.
  So the "-after.vim" script lives at ~/.vimuser/vimrc-after.vim instead
  of ~/.vimuser/someuser/vimrc-after.vim.

- ~/.vim/user/$VIMUSER-before.vim and ~/.vim/user/$VIMUSER-after.vim are
  still supported, but these locations are deprecated; instead,
  $VIMUSERFILES/vimrc-before.vim and $VIMUSERFILES/vimrc-after.vim are
  recommended.