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)

Add missing #include of err.h.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.

Convert errx(-1, ..., strerror(errno)) to err(-1, ...).

Discussed with: rwatson

Add support for tcpmd5 tests.
Add README.tcpmd5 to describe how to build a simple test setup
and run tests.

Convert compile time options to run time options [1].

Discussed with: rwatson
Suggested by: rwatson [1]

Add an #ifdef'd non-blocking version of the test.

Update copyright.

Use errx() instead of perror()/exit() for conciseness.

Suggested by: ru (some time ago)

Fixed a typo.

Submitted by: Pawel Worach

Add a simple C-based TCP connection generator, which generates and
closes the specified number of TCP connections sequentially and
synchronously. Useful for trying to trigger races in the accept
code.