salvager: Do not fork for single VG salvage

Currently we always fork a child in the salvager in order to salvage a
volume group. I believe this is in order to protect SEGV, exit(), etc
in one salvage operation from preventing salvaging anything else. When
salvaging a single volume group, though, there appears to be little
benefit.

In addition, we need to keep the VG salvaging code in the same process
as the cleanup code for single-volume salvages, so we can know which
volumes were deleted by SalvageVolumeGroup, so we know which volumes
to bring back online. So, do not fork for the singleVolumeNumber case.

Note that for DAFS, we already never fork for the entire salvage
operation when salvaging an individual volume group. So, this is
effectively a non-DAFS-only change.

Reviewed-on: http://gerrit.openafs.org/6787
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 46e85dd4682a3273e15bbf96c4d5492087555cab)

Change-Id: I545e9ad8d54ad63dbe8e221dfc6947446135cbb7
Reviewed-on: http://gerrit.openafs.org/9462
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>