Consolidate code for reading/writing vol headers

Consolidate code for reading/writing vol headers

The code for reading in and overwriting/replacing volume headers is
duplicated many times throughout the volume package. Consolidate the
code into the functions VReadVolumeHeader, VWriteVolumeHeader,
VCreateVolumeHeader, and VDestroyVolumeHeader. This makes it easy to
change the semantics of how headers are read/written, though this commit
should not change them.

Change-Id: Id395ee25fc2db92de9301b9cdbe18f30830e18d3
Reviewed-on: http://gerrit.openafs.org/1238
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>