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

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

Massively refactor ac(8).

- Use queue(3) -- not some homegrown implementation of linked lists.
- Rename structures to _entry, as they are entries in the linked list --
not the lists themselves.
- Don't store entire copies of struct utmpx in utmpx_entry, but only the
members we're interested in. Large fields such as hostnames are not
needed during the execution of the program.
- Give structure members useful names, instead of `name'.
- While there, use struct timevals instead of time_t's internally. This
is not strictly useful, but while we're at it...
- Mark stuff static.
- Add missing const keywords.
- Remove unneeded prototypes.
- Remove workaround for sparc64-specific utmp problems. These don't
apply to utmpx.
- Don't discard entries when timestamps are not monotone. This shouldn't
ever happen with utmpx, but discarding them is a bit too harsh.
- Remove debug code. We nowadays have `getent utmpx', which can be used
to analyze logfiles in depth.
- Use proper uppercasing/periods in comments.
- Print output of `ac -p' sorted alphabetically, instead of first
occurrence.
- Properly check against pts/* instead of tty[PQRSpqrs]* to determine
whether a TTY is a pseudo-terminal.

MFC after: 1 month

MFtbemd:

Prefer MACHNE_CPUARCH to MACHINE_ARCH in most contexts where you want
to test of all the CPUs of a given family conform.

The last big commit: let usr.sbin/ use WARNS=6 by default.

Compile 'ac' with DEBUG when arch==sparc64.

Fix indentation broken in previous __P removal.

Grumbled about by: bde

Perform a major cleanup of the usr.sbin Makefiles.
These are not perfectly in agreement with each other style-wise, but they
are orders of orders of magnitude more consistent style-wise than before.

Constify and set WARNS=2.

Submitted by: Mike Barcroft <mike@q9media.com>
Reviewed by: md5(1)

- Backout botched attempt to introduce MANSECT feature.
- MAN[1-9] -> MAN.

Set the default manual section for usr.sbin/ to 8.

$Id$ -> $FreeBSD$

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.

ac(8) utility, brought over from FreeBSD 1.1.5.