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

Explain more about the "C" qualifier for Celsius.

MFC after: 3 days

Document many of the sysctls in acpi. Bump the date on acpi_thermal.4
because I forgot to on the last change.

MFC after: 1 day

Add support for overriding the values for _CRT, _HOT, and _PSV via sysctl.
Prevent casual modification by requiring hw.acpi.thermal.user_override to
be set first. Fix printing of negative temperatures in the K->C conversion.
Document the remaining thermal sysctls.

MFC after: 3 days

Document passive cooling related sysctl MIB.

Fix typos and use consistent section names in manual pages:

s/SYSCTLS/SYSCTL VARIABLES/
s/TUNABLES/LOADER TUNABLES/
s/CAVEAT/CAVEATS/
s/DESCIPTION/DESCRIPTION/

Reviewed by: ru
MFC after: 3 days

Notify the user (at kern.emerg) that the system will be shutting down if
it is still above the critical temperature on the next poll cycle. This
is a 10 second advance notice by default. Document the private
(non-standard) notify we will be using with devd(8).

Some mdoc(7) polishing.

Correct _ACx description.
This is for a computer system, not for a furnace.

Add to the acpi_thermal man page, especially the notifies section.

Somebody called mdoc(7) janitor.

A bit of wordsmithing from a native speaker is still necessary.

Add acpi_thermal manual page,
because I asked about the unit of thermal
zone-related value too freqently.
Come on, mdoc police.