sys/net: New functions if_ioctl, if_init, and if_stop.

These are wrappers, suitable for inserting appropriate kasserts
regarding the API's locking contract, for the corresponding functions
in struct ifnet.

Since these are intended to commit configuration changes to the
interface, which may involve resetting the device, the caller should
hold IFNET_LOCK.  However, I can't straightforwardly prove that all
callers do yet, so the assertion is disabled for now.

(riastradh)