Adds apply_network_profile(), which looks up DNS servers for the entered
VLAN tag in lib/networks.conf and sets --nameserver accordingly — even
when IP is DHCP. Precedence: explicit env NAMESERVER > profile > static
prompt > inherit. Comma-separated DNS is normalised to spaces for pct.
Data file mapping a VLAN tag to its DNS servers and subnet, so build.func
can set the right resolvers from the tag entered at install time — even
with DHCP. New networks are a one-line addition here.