Only tun0 -> tun32767 may now be opened as struct ifnet's if_unit
is a short.
It's now possible to open /dev/tun and get a handle back for an available
tun device (use devname to find out what you got).
The implementation uses rman by popular demand (and against my judgement)
to track opened devices and uses the new dev_depends() to ensure that
all make_dev()d devices go away before the module is unloaded.
Reviewed by: phk
18d829816a