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.

MFH: sync netmap with the version in HEAD

MFC r239140: Clarify comments about number of tx / rx rings

MFC r239141: Fix whitespace (missing newline)

Sponsored by: ADARA Networks

MFC part of r238812 and remainder of r239149

From r238812, move mtx_init of per-ring locks into NIOCREGIF ioctl handler.
(Otherwise they're overwritten with zeros in netmap_if_new.)

From r239149:
Improve lock and unlock symmetry

- Move destruction of per-ring locks to netmap_dtor_locked to mirror the
initialization that happens in NIOCREGIF. Otherwise unloading a netmap-
capable interface that was never put into netmap mode would try to
mtx_destroy an uninitialized mutex, and panic.

MFC part of r239149:

Comment out the knlist_destroy for now as there is currently no
knlist_init.

Add missing mtx_destroy

This is a direct commit to stable/9 as the locking is somewhat different
in HEAD due to the VALE bridge work (r238812).

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.