vmx(4): add native netmap support

This change adds native netmap support for the vmx(4) adapter
(vmxnet3). Native support comes for free in 12, since the driver has been
ported to iflib. To make it minimally intrusive, the native support is
only enabled if vmxnet3.netmap_native is set at boot (e.g., in loader.conf).

Tested on stable/11 running inside vmplayer.

Submitted by: Giuseppe Lettieri <g.lettieri@iet.unipi.it>
Reviewed by: vmaffione, bryanv
Sponsored by: Sunny Valley Networks
Differential Revision: https://reviews.freebsd.org/D19104