Fix mac_interfaces use in loader.conf.

This change fixes the code in mfsbsd that expands the
mfsbsd.mac_interfaces variable set in loader.conf. Aside from a typo
that looked at cloned_interfaces, there was also the issue of
correctly quoting $i so that it expands $i rather than $i_mac.
Also need the ifconfig line in the rc.conf file.

The ifconfig_IF line is also needed in the interfaces file, otherwise
the interfaces startup script doesn't complete.
Fix nameserver setting in /etc/resolv.conf.

Whilst I'm here, also fix the setting of nameservers in
/etc/resolv.conf. Just a typo.
Merge pull request #21 from tdb/fix-kenv-mac-interfaces

Fix mac_interfaces use in loader.conf.