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

Print a warning if not setuid root.
Document the need for the setuid bit and how to set it.
Explain why it isn't set by default, and suggest simply adding users
to groups instead.

PR: docs/167741
MFC after: 3 weeks

Added the EXIT STATUS section where appropriate.

mdoc(7) police: one minor change that was not probably worth a commit.

Add the newgrp(1) utility, which changes groups. This is required by
the POSIX.2 UPE.

PR: 36190
Reviewed by: -standards, silence on -audit