Fix trying to lock an un-writable directory. Just put a message

Fix trying to lock an un-writable directory.  Just put a message
on the statusbar that we couldn't do it if the user modifies the file.
Changes to do_lockfile and write_lockfile to check for EACCESS and change
the return value of the functions (0 instead of -1)

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@4550 35c25a1d-7b9e-4130-9fde-d3aeb78583b8