Fix typo in macro and merge to error descriptions for the same error

New functions kthread_fpu_enter/exit.

The MI definitions don't do anything but maintain a flag, but MD code
can define kthread_fpu_enter/exit_md to actually enable/disable the
FPU. (These are almost pcu_load/discard on systems that use pcu(9),
except they apply to all PCUs.)

Discussed on tech-kern:
https://mail-index.netbsd.org/tech-kern/2020/06/20/msg026524.html

The proposed kthread flag KTHREAD_FPU is not included because I
couldn't find any particular need for it that would not be covered by
just calling kthread_fpu_enter/exit in the kthread function.

Remove the unused and all-but-unusable kthread_destroy(). Also makes
it more obvious that the opposite of kthread_create() is kthread_exit().

Rename slightly misleading KTHREAD_JOINABLE to KTHREAD_MUSTJOIN.

Improve wording.

Re-implement kthread_join(9), so that it actually works (hi haad@).

Remove boilerplate in CODE REFERENCES on file paths.
Describe in intro(9) how to read paths in the CODE REFERENCES section.

In order to widen the paragraphs, move the newly added list of flags below
the list of function parameters. Add two small sentences to give few more
clues about KTHREAD_JOINABLE. Add a missing prototype for kthread_join().
Also: newpl -> newlp.

Put flags into a list. Bug fixes to kthread_join docs.

Document kthread_join in kthread(9).

branches: 1.20.2;
Relate interfaces by updating "see also" section.

Mark up NULL and SCHED_*.

- Allow creating timeshard kthreads. To be used to fix the RAIDframe
parity rewrite issue.
- Create kthreads in the SCHED_RR class by default, not SCHED_FIFO.

branches: 1.17.4;
Convert TNF licenses to new 2 clause variant

branches: 1.16.6; 1.16.8;
kthread_create1 is no more, ok thx.

branches: 1.15.4;
Sync with reality.

Bump date for previous.

Mention that a kthread *must* call kthread_exit() to terminate itself
properly. If you don't, the kernel will likely crash.

branches: 1.12.2;
s/the the/the/ (only in sources that aren't regularly imported from
elsewhere).

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.

"Utilize" has exactly the same meaning as "use," but it is more
difficult to read and understand. Most manuals of English style
therefore say that you should use "use".

New sentence, new line.

fix typo. PR 18597
put full ANSI prototype in FUNCTIONS section for consistency.

Fix the declaration of the function to be registered with kthread_create{,1};
from Peter Seebach in PR kern/15838.

Generate <>& symbolically. I'm avoiding .../dist/... directories for now.

proc0 isn't the init.

fix typo (shaed)

Two punctuation nits, and sort SEE ALSO.

Boring whitespace fixes.

Add some missing documentation.