vlserver: do not perform ChangeAddr on mh entries, except for removal

vlserver: do not perform ChangeAddr on mh entries, except for removal

Fix a long standing bug in the ChangeAddr RPC which damages the vldb,

When vos changeaddr is run with -oldaddr and -newaddr, and the -oldaddr
is present in an multi-homed entry, instead of changing the address in
the mh entry, the server slot is "downgraded" to a single homed entry
and the mh entry is orphaned in the vldb.

Instead, if the -oldaddr is in a multi-home entry, refuse to change the
address with a VL entry not found error and log the event.

Multi-homed addresses can be changed manually using the vos setaddrs
command which calls the RegisterAddrs() RPC.

Change-Id: I20ba3e7fa5ffdb1b1abd2e2a716d8e4bb6594542
Reviewed-on: http://gerrit.openafs.org/11639
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>