netstat(1): increase width of Netif column

The previous width of Netif (10 or 8) was too short for modern interface
names; make it 12, which is long enough to display "epair0a.1000".

This came up in practice with genet(4) interfaces, since the base
interface name is long enough that with the previous limit, VLAN
identifiers would be truncated at 1 character in the IPv6 output:
"genet0.100" becomes "genet0.1".

The width is now fixed, and doesn't depend on the address family,
because there's no reason that length of the interface name would vary
based on the AF.

Reviewed by: imp,zlei,Mina Galić
Pull Request: https://github.com/freebsd/freebsd-src/pull/1223