Add if_inc_counter(), a generic method to update ifnet(9) counter
w/o dereferencing the struct.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
While not too late rename if_get_counter_compat() to if_get_counter_default().
The compat counters will go away, but the function will remain in its place,
and in all places where it is going to be called.

Discussed with:	melifaro