Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

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

Suggested by: ru (some time ago)

Add tcpstream, a simple TCP stream generator that uses a pseudo-random
sequence to detect data corruption visible to an application.