Use draft folder petname if defined

* wl-draft.el (wl-draft-save, wl-draft-reedit,
  wl-draft-buffer-change-number): Use draft folder petname if defined.
* wl-util.el (wl-collect-draft): Use draft folder petname if defined.

Patch provided by Markus Huber in
<mid:2014052108:51:07markus.huber@informatik.uni-augsburg.de>.
Update elmo-file so it works with modern versions of the file utility.

This uses "file -i" instead of "file -m".  Additionally, if file is kind
enough to provide a charset, we trust the result and avoid decoding and
recoding the data multiple times.
Fix typo in elmo-map-folder-list-message-locations (elmo-file-folder).

Due to a missing expand-file-name, we were including subdirectories,
which caused searches to fail.