Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add const qualifier to the dst parameter of the ifnet if_output method.

Remove if_timer/if_watchdog now that they are no longer used. The space
used by if_timer is reserved for expanding if_index to an int in the
future.

Reviewed by: rwatson, brooks

During if_detach(), invoke if_dead() to set the ifnet's function
pointers to "dead" implementations that no-op rather than invoking
the device driver. This would generally be unexpected and
possibly quite badly handled by most device drivers after
if_detach() has completed.

Reviewed by: bms
MFC after: 3 weeks