findmnt: revert --fstab behavior, add --tab-file option

findmnt: revert --fstab behavior, add --tab-file option

The optional argument for -s, --fstab options (--fstab=file) is not
backwardly compatible, because command line with short options (e.g.
findmnt -snr) will be interpreted incorrectly.

This patch removes support for the optional --fstab argument and
introduces a new option --tab-file, for example:

	findmnt --fstab --tab-file=/etc/fstab.foo

Reported-by: Juergen Daubert <jue@jue.li>
Signed-off-by: Karel Zak <kzak@redhat.com>