mount: add shortoptions for bind, move and rbind

mount: add shortoptions for bind, move and rbind

Reserving uppercase letters for mount operations:

    --move  | -M
    --bind  | -B
    --rbind | -R

Add lowercase for the most needed mount operation that happen
in initramfs: mount -M /sys /root/sys

Note, we still have shared-subtree operations (--make-{slave,private,...})
without short options.

[kzak@redhat.com: minor change in mount.8]

Signed-off-by: maximilian attems <max@stro.at>
Signed-off-by: Karel Zak <kzak@redhat.com>