doc: backup manpage fixes

 - Consistently specify -dryrun and -n across various subcommands.
   Many did not list -n, some listed -n but no -dryrun, and some
   listed -noexecute instead.

 - backup_volrestore: Add missing option -usedump

 - backup_deletedump: Add missing options -groupid, -dbonly, -force,
   and -portoffset

Change-Id: Iec1c36cba0ad0e61d7e6215c9cba81228b95a81f
Reviewed-on: http://gerrit.openafs.org/10441
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>