Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFH: sync the netmap code with the one in HEAD
(enhanced VALE switch, netmap pipes, emulated netmap mode).
See details in the log for svn 261909.

Merge from head: sync the netmap code with the one in HEAD

MFC: sync the version of netmap with the one in HEAD, including device
drivers (mostly simplifying the code in the interrupt handlers).

On passing, also merge r250414, which is related to netmap
and the use of lem/em in virtual machines.

MFC some flags and minor things related to "transparent mode"
that i forgot in previous commits.

MFH: sync netmap with the version in HEAD

MFC: the netmap code from HEAD, now supported in the ixgbe/ and e1000/
drivers (re/ will come in the next commit)

MFC: import netmap core files into RELENG_9.
This is the same code as in HEAD.

Device driver modifications will be imported separately
because the base drivers differ and patches might be
slightly different between the various releases.

The code is disconnected from the main build targets
unless you explicitly put a 'device netmap' in your
kernel config file.