Fail if the user asks for --enable-{sniffing type} and we can't provide it.

Treat --enable-{sniffing type} as a demand that we provide it; if we
can't do so, fail, don't just warn about its absence (with a message
that might well get lost amongst the other configure-script output) and
drive on.  If they don't demand it, provide it if we can, warn and drive
on if we can't.
Don't fail if --enable-can wasn't specified and we can't support CAN sniffing.
Return the right figure of merit for the "any" device.

Thanks and a tip of the Hatlo hat to Coverity Scan for picking up this
one; we were picking the right value for "any", and then overriding it
with a value determined by parsing the name.
Add the UP and RUNNING flags to the "any" device.

That way, code that looks at those flags will make the "any" device look
like regular devices; as long as there's at least one network interface
up and running, or could be up and running at some point in the future,
the "any" device can be considered up and running, so that's
appropriate.