We may want to force a rtnet interface up, and more likely down before
unloading a driver module.

Export the ioctl() code which handles the IOC_RT_IFUP/IFDOWN core
ioctl requests as standalone routines. The locking model remains
unchanged, with ioctl() requests and direct service calls serializing
on the per-device lock.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>