ithread(9): update functions to current day

ithread(9): update functions to current day

The public KPI is now intr_event_**,
 - Convert existing documented functions to their equivalents.
 - Fix up the function arguments
 - Fix up the possible error return values for each
 - Remove ithread_schedule() completely
 - Rename man page to intr_event(9)
 - Update cross-references

Future changes will update the descriptive text for these functions.

PR:		100803
Based on work by: trhodes
Reviewed by:	jhb
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D33475

(cherry picked from commit 3cdbaee3548a6caa3ab7aca5788775057b1bd1fd)