Wheezy's NM automatically creates connections for all wired interfaces
that match against the current (possibly spoofed) MAC address, but
only matches them against the permanent MAC address when trying to
auto-connect. Hence, if MAC spoofing is enabled, no wired connection
will match, and there's no auto-connection. We work around this by
adding a system connection that doesn't specify a MAC address so it
matches all interfaces.

This closes #6911.