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

MFC r326744:

Correctly report the vmxnet3 link down media status

MFC r318867: fix vmxnet3 crash when LRO is enabled

The crash can occur when all of the following conditions are true:
- a packet consists of multiple segments (requires LRO enabled)
- there has been a failure to allocate an mbuf for the packet and
the packet has to be dropped
- a host (vmware) still owned at least one segment of the packet,
so the driver had to wait for another interrupt to proceed to
discarding the remaining segment(s)

Reviewed by: rstone
Approved by: re (gjb)
Sponsored by: Panzura

Update if_hwassist when interface options are changed.

In particular for me this fixes checksum problem when if_bridge attached
to the interface requests TXCSUM to be disabled, but effectively ignored.

Approved by: re (kib)