Redocument xc_barrier.

Be more clearer, and more correcter, about what it does.

Document xc_barrier.

Use .Bl -tag -width Fn. There's no such thing as -width compact
or, rather, there is, but the width specified is that of the word
"compact".

Avoid using .Fn for XC_HIGHPRI_IPL as it looks inconsistent with
.Dv XC_HIGHPRI

Format (*func)(arg1, arg2) so that "func" too is formatted as an
argument (the incantation is, admittedly, rather baroque and kludgy).

branches: 1.12.4;
Support arbitrary softint IPLs in high priority xcall

The high priority xcall supported only a softint of IPL_SOFTSERIAL. It meant
that it didn't work for xcall callbacks depending on lower IPLs than
IPL_SOFTSERIAL.

The change makes xcall have multiple softints of IPLs and allow users to specify
arbitrary IPLs. Users can specify an IPL by XC_HIGHPRI_IPL passed to the 1st
argument of xc_broadcast or xc_unicast.

Note that xcall still serves requests one by one (i.e., doesn't run them
concurrently) even if requests have different IPLs.

Proposed on tech-kern@

Refer softint(9)

branches: 1.10.18;
Update xcall(9).

Use Mt for email addresses.

branches: 1.8.2; 1.8.8; 1.8.12;
- document XC_HIGHPRI
- explicitly note that xc_wait requires thread context.

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

IMPLEMENTATION NOTES back to NOTES.

Use standard section headers.

branches: 1.4.2;
Fix Dt, remove trailing whitespace, sort SEE ALSO.

xcall(9): correct the point about preemption, add NOTES, do not mentioned
some implementation details.

Remove xc_init_cpu(), percpu_init(), percpu_init_cpu() - they are part
of implementation, not public API. OK dyoung@.

Rename xc(9) to xcall(9). OK dyoung@.