libafs: afs_InitVolSlot function

Add a new local function to initialize newly gotten volume slots and
move that code out of afs_SetupVolume().  Initialize the slot before
putting the volume in the volume hash table list.

Make it more clear to avoid using record 0. The volume items record 0 is
not used, so avoid setting the tf pointer to the static fvolume buffer
when reading record 0.

Change-Id: Iffba52fbf8d72459c9a36015964e61d485f22ad4
Reviewed-on: http://gerrit.openafs.org/10877
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>