file-systems: Do not truncate mount points that already exist.

Reported by David Thompson <dthompson2@worcester.edu>
at <https://lists.gnu.org/archive/html/guix-devel/2015-10/msg00284.html>.

* gnu/build/file-systems.scm (mount-file-system): When SOURCE matches
  'regular-file?', do not create MOUNT-POINT if it already exists.  This
  fixes a bug whereby we would be truncating MOUNT-POINT if it already
  existed.