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.

MFC r233648:
Remove trailing whitespace per mdoc lint warning

Approved by: cperciva (implicit)

MFC r235317:

General mdoc(7) and typo fixes.

PR: 167776

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.