LU-9859 libcfs: add "default" keyword for debug mask

Allow "lctl set_param debug=default" to reset the debug mask to
the default value.  This is useful if the debug needs to be set
to a higher value temporarily, but should be easily reset back
to the original value afterward.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I7d0d8fb81e51afb5ea6f29abea0d0814de3ebbe5
Reviewed-on: https://review.whamcloud.com/46251
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>