rbdmap: per-device mount (Closes: #8538)

`/etc/init.d/rbdmap start` was doing `mount -a`. Although (arguably)
`mount -a -O _netdev` could be less disruptive, it's not RBD mapping job to
mount unrelated devices and potentially do it at the wrong time.

Solution is to call `mount {device}` which works as expected and mounts
device even if it given in form `mount /dev/rbd/pool/imagename` while
`/etc/fstab` uses UUID or LABEL notation.

Furthermore this commit

 * fixes global exit code (it was always 0): now it is 0 only when
   all devices were (un)mounted successfully; otherwise non-zero.
 * replaces `mount -a` with per-device post-mapping `mount {dev}`
 * show mapping progress using LSB functions per device instead of for
   {start|stop} invocation.
 * capture output of `(u)mount` (if any) and report it as "info".

Signed-off-by: Dmitry Smirnov <onlyjob@member.fsf.org>