freebsd_network & drivers: Remove need to specify FBSD_TASKQUEUES.

Instead make FBSD_TASKQUEUES a combination flag of all taskqueues,
so that if even one is needed, the init_taskqueues function will
be invoked appropriately.

drivers/network: Move ethernet drivers to an "ether" subdirectory.

Now that there is also a "tap" driver in here, and the WiFi drivers
are also in a subdirectory, moving the physical ethernet device drivers
to a subdirectory also makes sense.

(Briefly) discussed on the mailing lists.