Build a separate copy of vlib for dasalvager

Build a separate copy of vlib for dasalvager

Currently dasalvager links to vlib.a. But vlib.a is built without any
DAFS defines, and so the size of a struct DiskPartition64 is different
(since dasalvager is built with AFS_DEMAND_ATTACH_UTIL). Build our own
copies of the volume package files instead, with
AFS_DEMAND_ATTACH_UTIL defined.

Change-Id: I7b9f965fc18b6e6b71275912684ad041aaef19f4
Reviewed-on: http://gerrit.openafs.org/4875
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>