Test for DHCP interfaces explicitly.

If there are no nameserver set the code that generates resolv.conf
exits without generating a resolv.conf.  This is done to
keep from stomping a resolv.conf that is generated from an interface
that is configured via DHCP, however this can cause the script to
bail out and leave an old incorrect resolv.conf in place if you
simply delete the nameservers from the GUI when you only have
statically configured interfaces. (although why you
would want to do this isn't immediately obvious)

Ticket: #7293