Disable interrupts while in kdb_trap() to handle cases where the caller

Disable interrupts while in kdb_trap() to handle cases where the caller
doesn't do it.  This fixes all known causes of "Context switches not
allowed in the debugger" in mi_switch().  The main cause was trap_fatal()
calling kdb_trap() with interrupts enabled.  Switching to ithreads for
interrupt handling then made fatal traps more fatal and harder to debug.
The problem was limited in -current because most interrupt handlers are
blocked by Giant, but it occurred almost deterministically for me because
my clock interrupt handlers are non-fast and not blocked by Giant.