(input_update): minor optimization: don't call str_term_width2() twice.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Sync with gnulib 47f4ca8fe9506254200028c8a8c5fffbf1bf1161

Don't consider "devtmpfs" as dummy.

* src/filemanager/mountlist.c (ME_DUMMY_0): Remove "devtmpfs"
as there is storage associcated with it.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>