Use Xr instead of Fn to refer kqueue(2)

Use Xr instead of Fn to reference splsched(9)

branches: 1.15.8; 1.15.12;
Merge riastradh-drm2 to HEAD.

Use Mt for email addresses.

branches: 1.13.2; 1.13.8; 1.13.14; 1.13.18;
Use .In instead of .Aq Pa for header files.

Convert TNF licenses to new 2 clause variant

branches: 1.11.32; 1.11.34;
bump date for previous

Remove duplicate word. From Ognyan Kulev in PR 24472.

Use
.In header.h
instead of
.Fd #include \*[Lt]header.h\*[Gt]
Much easier to read and write, and supported by groff for ages.
Okayed by ross.

Bump date and copyright for last.

Noet that knote() must not be called from interrupt contexts running at an
interrupt priority level higher than splsched().

g/c the NEXTRELEASE comment tag now that the next version number is decided

update HISTORY - kqueue branch was merged
bump date

KNOTE() check if the list is nonempty now

misc mdoc fixes.

bring knote(9) and kfilter_register(9) from kqueue branch to mainline
HISTORY section mentions the interface is only available on experimental
kqueue branch (for now)

branches: 1.1.2;
file knote.9 was initially added on branch kqueue.