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

Replace log(3) with flsll(3) for watchdogd(8) and drop libm dependency.

MFC after: 2 weeks

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

Use pidfile(3) in watchdogd(8).

Rename the WATCHDOG option to SW_WATCHDOG and make it use the
generic watchdoc(9) interface.

Make watchdogd(8) perform as watchdog(8) as well, and make it
possible to specify a check command to run, timeout and sleep
periods.

Update watchdog(4) to talk about the generic interface and add
new watchdog(8) page.

style.Makefile(5):
Use WARNS?= instead of WARNS=.

o style(9) fixes
- Reordered #includes
- Only include <sys/types.h>, not it and <sys/cdefs.h>
o style.Makefile(5) fixes
- No SRCS= line when only one src file with same name as program
o Use warn()/errx() instead of fprintf()
- Integrated patch from Philippe Charnier <charnier@xp11.frmug.org>

Approved by: jeff (mentor)

- Add a software watchdog facility.

This commit has two pieces. One half is the watchdog kernel code which lives
primarily in hardclock() in sys/kern/kern_clock.c. The other half is a userland
daemon which, when run, will keep the watchdog from firing while the userland
is intact and functioning.

Approved by: jeff (mentor)