p910nd: init: add txtvers=1 to mDNS properties

p910nd: init: add txtvers=1 to mDNS properties

The spec https://developer.apple.com/bonjour/printing-specification/bonjourprinting-1.2.1.pdf
notes:

... if the meaning of any of the TXT record keys is changed, the txtvers value
will be incremented. The current value of this key is “1”, and if this key does not exist in
the TXT record, the default value of “1” is assumed. The txtvers SHOULD be the first
key/value pair in the TXT record.

Signed-off-by: Paul Donald <newtwen@gmail.com>