mkfs: add string options to generic parsing

mkfs: add string options to generic parsing

So that string options are correctly detected for conflicts and
respecification, add a getstr() function that modifies the option
tables appropriately.

[ Eric Sandeen <sandeen@redaht.com>: whitespace fixup ]

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jan Tulak <jtulak@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>