bug#13870: recentf-save-file defaults to the wrong directory

[Josh]

The defcustom form for `recentf-save-file' contains a standard value
of (convert-standard-filename "~/.recentf"), but additional per-user
Emacs-specific files should be stored under `user-emacs-directory'
according to its docstring.

In GNU Emacs 24.3.50.1 (x86_64-apple-darwin, NS apple-appkit-1187.34)
 of 2013-03-04 on bix
Bzr revision: 111935 yamaoka@jpl.org-20130304102733-4qy111z41qwoh2as
Windowing system distributor `Apple', version 10.3.1187
Configured using: `configure --host=x86_64-apple-darwin
--build=i686-apple-darwin --with-ns'
[...]