RW Replication: Add basic definitions

RW Replication: Add basic definitions

Add some basic definitions that will be needed to handle RW
replicas.

A new volume type RWREPL is added.  Replicas will share the same
volume ID as the RW volume, so the array of volume IDs by volume
type is unchanged, as is the VLDB entry format.

A new flag bit ITSRWREPL/VLSF_RWREPLICA for serverFlags identifies
RW replica sites in VLDB entries.

Change-Id: I882b238f34e682ebea782e11dc418ae1340d4546
Reviewed-on: http://gerrit.openafs.org/6676
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>