Add some additional empty string checks for IPv4 and try to configure
a netmask along with the address rather than doing things twice.
Contrary to AUTO-DHCP, IPv6-SLAAC will accept static configuration
as well, which we will use at least for resolv.conf currently and
if we were given a static address configure that as an alias as well.
The pc-sysinstaller changes going along were committed to PC-BSD as r10773.
Reviewed by: kmoore
Sponsored by: The FreeBSD Foundation
Sponsored by: iXsystems
MFC after: 20 days
ca3458c071