While "jng" or "jib" without arguments told you what each sub-command does,
sub-command usage didn't tell you (e.g., "jng bridge" or "jib addm" gave
only usage and not description).
Reduce differences between jib/jng and fix a bug that would prevent
additional interfaces from being created if the first of many already
existed (counter wasn't incremented before calling only continue).
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 `='.
Documented in iwn(4), "Only one virtual interface may be configured at any
time." However, netgraph with a cloned MAC address is able to communicate
over an ng_eiface attached to an ng_bridge linked to the wlan(4) interface.
While here, introduce syntax to specify the MAC address is to be cloned if
the named interface begins with equals [=].
