ifconfig(8): Fix support for automatic driver module loading.

ifconfig(8): Fix support for automatic driver module loading.

Before this commit, trying to configure for example 'ifconfig vxnet ..."
would have kldload'ed the if_vx module even if there was a if_vxnet one
if they both existed and none of them was loaded.

FreeBSD fixed this in r279951. This commit brings in their ifmaybeload()
function wholesale.

Taken-from:  FreeBSD
Reported-by: Bill Yuan <bycn82@leaf.dragonflybsd.org>