Use same interrupt vector handler for fast/slow interrupt handlers

Use same interrupt vector handler for fast/slow interrupt handlers

Slow interrupt vector handler is removed.  Fast interrupt vector handler,
ithread_fast_handler(), now schedules slow interrupt handlers if necessary:
o  No fast interrupt handlers are registered
o  Mixed fast and slow interrpt handlers are registered
o  Non-MPSAFE fast interrupt handlers could not get BGL

i386/amd64: gd_ipending field in mdglobaldata is revoked, which is only
used by slow interrupt vector handler.

ithread_fast_handler()'s invoking convetion is changed:
- ithead_fast_handler() must be called with critical section being held
- Callers of ithead_fast_handler() no longer bump gd_intr_nesting_level

Discussed-with: dillon@
Reviewed-by: dillon@