ifnet: make if_index global

Now that ifindex is static to if.c we can unvirtualize it. For lifetime
of an ifnet its index never changes. To avoid leaking foreign interfaces
the net.link.generic.system.ifcount sysctl and the ifnet_byindex() KPI
filter their returned value on curvnet. Since if_vmove() no longer
changes the if_index, inline ifindex_alloc() and ifindex_free() into
if_alloc() and if_free() respectively.

API wise the only change is that now minimum interface index can be
greater than 1. The holes in interface indexes were always allowed.

Reviewed by: kp
Differential revision: https://reviews.freebsd.org/D33672

(cherry picked from commit 91f44749c6feb50f39af8805dd803e860f0418f1)