in input_tab(), disable completion of usernames, directories, and

in input_tab(), disable completion of usernames, directories, and
filenames if the cursor isn't at the end of the line, as it can lead to
odd behavior (e.g. adding a copy of the entire match to the middle of
the line instead of just the uncompleted part of the match)

git-svn-id: svn://svn.savannah.gnu.org/nano/branches/nano_2_0_branch/nano@4176 35c25a1d-7b9e-4130-9fde-d3aeb78583b8