resolve_ipnets(): Repair family prefix handling.

When we changed how we handled the case when no IP family flags were
passed to resolve_ipnets() some days ago, we inadvertently broke the
handling of explicit family prefix in the network specification (i.e,
the "ipvX:" part of e.g. "ipv6:hostname"), as the ipfamilies parameter
of the resolve_ipnets__netspec() helper would no longer contain either
"ipv4" or "ipv6", thus resulting in an the error "IP family mismatch"
being raised.

Add special handling of this case.

This was originally commit 4e674f8cc29f in nsc-puppet-utils.