- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

Handle NULL return from crypt(3). Mostly from DragonFly

Fix a memory leak in addgroup() by ensuring the allocated memory
is freed if an error occurs.

PR: bin/161510
MFC after: 4 weeks

Partially revert r226274, the old code was right.

- Fix an off-by-one bug in addgroup().
- Fix the error message when setgid() failed.

PR: bin/161509
Submitted by: Jeremy Huddleston <jeremyhu apple com>
MFC after: 2 weeks

Revert most part of 200420 as requested, as more review and polish is
needed.

Remove unneeded header includes from usr.bin/ except contributed code.

Tested with: make universe

In preparation for raising NGROUPS and NGROUPS_MAX, change base
system callers of getgroups(), getgrouplist(), and setgroups() to
allocate buffers dynamically. Specifically, allocate a buffer of size
sysconf(_SC_NGROUPS_MAX)+1 (+2 in a few cases to allow for overflow).

This (or similar gymnastics) is required for the code to actually follow
the POSIX.1-2008 specification where {NGROUPS_MAX} may differ at runtime
and where getgroups may return {NGROUPS_MAX}+1 results on systems like
FreeBSD which include the primary group.

In id(1), don't pointlessly add the primary group to the list of all
groups, it is always the first result from getgroups(). In principle
the old code was more portable, but this was only done in one of the two
places where getgroups() was called to the overall effect was pointless.

Document the actual POSIX requirements in the getgroups(2) and
setgroups(2) manpages. We do not yet support a dynamic NGROUPS, but we
may in the future.

MFC after: 2 weeks

Use (char *)NULL to terminated to argument list for execl().
Without this cast the compiler cannot know that it has to convert
the null pointer constant NULL to a null pointer.

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

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