bos: re-add -salvagedirs for use with -all

bos: re-add -salvagedirs for use with -all

The MR-AFS support code had a -salvagedirs option that was passed
through to the salvager (when running, and when -all was used),
that was removed in commit a9301cd2dc1a875337f04751e38bba6f1da7ed32
along with the rest of the MR-AFS commands and options.

However, it is useful in its own right, so add it back and allow
the use of -salvagedirs -all to rebuild every directory on the server.

Change-Id: Ifc9c0e4046bf049fe04106aec5cad57d335475e3
Reviewed-on: https://gerrit.openafs.org/12457
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>