(show_dev): Omit local devices if show_all_fs is negative.

(show_dev): Omit local devices if show_all_fs is negative.
(show_all_fs): If negative, omit non-local filesystems.
All uses of (all_fs != 0) changed to (all_fs > 0).
(long_options, usage, main): Add -l or --local option.
(main): When asking for df of an explicit file name, get all
the mount points, so that we're more likely to find it when
we look it up.