kern/intr: start adding non-NULL event interface variant

[ehem_freebsd_m5p.com]

Summary

Nearly all callers have are known to have already initialize the event
before calling these functions. As such add variants which assume the
event will be non-NULL and will panic() if this occurs.

[ehem_freebsd_m5p.com]

@mhorne this is a follow-on to D33479. If the normal interface is going to include NULL-checks, but they can be omitted in some fairly common circumstances what do you prefer for new names? Underscore prefix tends to point to internal (though can also indicate lower-level), but some letter suffix would also work. Your view since you've been emphasizing the consistency here?