Rename global ifnet TAILQ

Rename global ifnet TAILQ

Naming the list like the struct itself makes for awful grepping.
Call the global variable "ifnetlist" from now on.

There used to be kvm(3) consumers in base picking up this symbol, but those
have long been converted to other interfaces.

A few potential ports users remain, same deal as sys/net/if_var.h r1.116
"Remove struct ifnet's unused if_switchport member":  they get bumped.

Previous users pointed out by deraadt
OK bluhm