MFC r326276:

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Add the SUSv3 -l option to at. This is an alias for atq. Allow job ids
to be specified on the command line for which information should be reported.

Submitted by: Joe Halpin <joe.halpin@attbi.com>
Reviewed by: mike

o Remove the -V [version number] option, since our version of at(1) no
longer resembles the original.
o Remove references to `you' in the manual.

Submitted by: Joe Halpin <joe.halpin@attbi.com>

Implement the POSIX 1003.1-2001 -r and -t options in at(1). Fix some
minor issues in the rest of the source and manual.

Submitted by: Joe Halpin <joe.halpin@attbi.com>
Obtained from: touch(1) (partially)
MFC after: 1 month

Style improvements recommended by Bruce as a follow up to some
of the recent WARNS commits. The idea is:

1) FreeBSD id tags should follow vendor tags.
2) Vendor tags should not be compiled (though copyrights probably should).
3) There should be no blank line between including cdefs and __FBSDIF.

Use __FBSDID()

WARNS=2 fixup.

SECURITY.

Fixed macros for temporarily relinquishing and restoring setuid/setgid
privileges so that they never change the real user and group IDs of
the calling process.

The setre[ug]id() calls are still used in the REDUCE_PERM macro (with
the r[ug]id arguments of -1) so that the call changes the saved user
and group IDs of the process to that specified.

Also, the panic() and perr() functions had insufficient privileges to
delete the problematic file under /var/at.

Correct use of .Nm, .Em, .Ev
Add rcsid. Use errx instead of fprintf + exit.
Various spelling fixes.

$Id$ -> $FreeBSD$

Eliminate compiler warnings from -Wall

Use err(3).
Simplification: if(a == 12) a -= 12 -> if(a == 12) a = 0.

Revert $FreeBSD$ to $Id$

Make the long-awaited change from $Id$ to $FreeBSD$

This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.

Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.

Upgrade to 2.9

Remove trailing whitespace.

Upgrade.

Added at/atrm/atq/batch from Linux as hacked by Chris Demetriou.