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

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Teach the loopback interface about checksum generation and validation
avoidance:

- Enable setting the RXCSUM and TXCSUM flags for loopback interfaces;
set both by default.
- When RXCSUM is set, flag packets sent over the loopback interface as
having checked and valid IP, UDP, TCP checksums so that higher
protocol layers won't check them.
- Always clear CSUM_{IP,UDP_TCP} checksum required flags on transmit,
as they will have gotten there as a result of TXCSUM being set.

This is done only for packets explicitly sent over the loopback, not
simulated loopback via if_simloop() due to !SIMPLEX interfaces, etc.

Note that enabling TXCSUM but not RXCSUM will lead to unhappiness, as
checksums won't be generated but will be validated.

Kris reports that this leads to significant performance improvements
in loopback benchmarking with TCP and UDP for throughput:

RXCSUM RXCSUM+TXCSUM
TCP 15% 37%
UDP 10% 74%

Update man page.

Reviewed by: sam
Tested by: kris
MFC after: 1 week

mdoc(7) police: removed HISTORY info from the .Os call.

pseudo-device -> device in kernel config lines. Removed whitespace at EOL.
Reviewed by: joerg, dd

$Id$ -> $FreeBSD$

Use Cd macro for SYNOPSIS.

PR: docs/10495

Revert $FreeBSD$ back to $Id$

Make the long-awaited change from $Id$ to $FreeBSD$

This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.

Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.

Sort cross references.

Do not install the now unsupported netns and netiso
related man pages. Comment out cross references to those man
pages from other man pages.

This commit was generated by cvs2svn to compensate for changes in r1638,
which included commits to RCS files with non-trunk default branches.

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