LU-16767 mdt: Allow jobID fields widths

LU-16767 mdt: Allow jobID fields widths

Modify the function jobid_interpret_string in order to allow admin to
specify the widths of parameter printed by
lctl get_param mdt.*.job_stats.
By specifying the parameter jobid_name, the admin can truncate the
fields.
For exemaple, the format "%3e.%u.%6h" will print in job_stats
the 3 first characters of the executable name, a dot, the whole uid, and
the 6 first characters of the hostname.
If no digit is passed before the letter, it will print the whole field.

Signed-off-by: Giardi Sylwyn <sylwyn.giardi@cea.fr>
Change-Id: Ifd94b354cef07a7fff5e70c94c313a7e4617e2f8
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58822
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>