set "multi-master" automatically with multiple primaries

set "multi-master" automatically with multiple primaries

By default, named logs a message at level info if a secondary
zone receives an update indicating that the serial number has
gone backwards. The "multi-master" option was provided to
allow this message to be suppressed if a zone was configured
with multiple primary servers.

That option has now been marked obsolete. The message is now
logged at debug level 1 when there are multiple primary server
addresses configured for the zone. It is still logged at level
info if there is only address.