tune2fs: update journal users while updating fs UUID (with external journal)

When we have fs with external journal device, and updating it's UUID, we
should update UUID in users list for that external journal device.

Before:
$ tune2fs -U clear /tmp/dev
tune2fs 1.42.10 (18-May-2014)
$ dumpe2fs /tmp/dev | fgrep UUID
dumpe2fs 1.42.10 (18-May-2014)
Filesystem UUID:          <none>
Journal UUID:             da1f2ed0-60f6-aaaa-92fd-738701418523
$ dumpe2fs /tmp/journal | fgrep users -A10
dumpe2fs 1.42.10 (18-May-2014)
Journal number of users:  2
Journal users:            0707762d-638e-4bc6-944e-ae8ee7a3359e
                          0ad849df-1041-4f0a-b1c1-2f949d6a1e37

After:
$ sudo tune2fs -U clear /tmp/dev
tune2fs 1.43-WIP (18-May-2014)
$ dumpe2fs /tmp/dev | fgrep UUID
dumpe2fs 1.42.10 (18-May-2014)
Filesystem UUID:          <none>
Journal UUID:             da1f2ed0-60f6-aaaa-92fd-738701418523
$ dumpe2fs /tmp/journal | fgrep users -A10
dumpe2fs 1.42.10 (18-May-2014)
Journal number of users:  2
Journal users:            0707762d-638e-4bc6-944e-ae8ee7a3359e
                          00000000-0000-0000-0000-000000000000

Also add some consts to avoid *magic numbers*:
- UUID_STR_SIZE
- UUID_SIZE
- JFS_USERS_MAX
- JFS_USERS_SIZE

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>