Make backend's bonjour service name dns-compliant

The master/slave label has been moved into the TXT record to reduce
the service name below the 15 character limit for DNS records. Per RFC 6335

This may break compatibility with some existing clients, but it's necessary
to make it work with many more clients.

Fixes #10773

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>