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

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

MFC r243238:
Standardize EXIT STATUS instructions in man pages when possible.

Approved by: cperciva (implicit)

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

Approved by: re (implicit)

Add seq(1), a small utility to generate sequence number.

Obtained from: NetBSD
MFC after: 3 months