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