Add syntax to disable MAC allocation

Adding `!' before an interface name will disable MAC allocation, falling
back to driver mechanics. Alternatively adding `=' before an interface name
causes the MAC address to be cloned (for ng_bridge(4) back-end only). While
here, disable the auto-detection of wlan* since this knocks the host off;
requiring the host that defines the jail to explicitly enable this feature
by preceding the interface with `='.