rc: network.subr improve network6_getladdr()

In network6_getladdr() we are iterating over inet6 lines and are not
interested in any others. So tell ifconfig to limit output to "inet6"
as much as possible.
This is probably a micro-optimisation but was noticed while looking
at other IPv6-related boot-time improvements.

(cherry picked from commit e448ff6f0e8035c3dd5b7bd0872f3f0d91c4e0d0)