LU-13055 mdd: per-user changelog names and mask

LU-13055 mdd: per-user changelog names and mask

Allow specifying a name for newly-registered changelog users,
rather than the default "clNNN" that is otherwise used. This
allows services to register a "well-known" changelog user,
rather than having to store the changelog username in HA storage
outside of the filesystem.

Each changelog user still has a unique ID appended to it, to allow
the changelog_clear and changelog_deregister commands to be run
using only the ID if necessary/desired. User name can be used to
deregister. User name is also unique per server.

If no name is given, then default "cl" format is used.

With this new functionality, it is possible to specify the name like:
 # lctl --device testfs-MDT0000 changelog_register --user watcher
   testfs-MDT0000: Registered changelog userid 'cl13-watcher'

Per-user mask is also added to allow specific operation logging on
per-user basis. Mask can be set only during registration. Resulting
mask from per-server mask and all user masks is used for current
changelog operations.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I56028f54cc97bbc9af03fd6559c19ef854f759d8
Reviewed-on: https://review.whamcloud.com/43380
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>