Pass the trap type and code down from db_trap() to db_stop_at_pc() so

Pass the trap type and code down from db_trap() to db_stop_at_pc() so
that the latter can easily determine what the trap type actually is
after callers are fixed to encode the type unambigously.

ddb currently barely understands breakpoints, and it treats all
non-breakpoints as single-step traps.  This works OK for stopping
after every instruction when single-stepping, but is broken for
single-stepping with a count > 1 (especially with a large count).
ddb needs to stop on the first non-single-step trap while single-
stepping.  Otherwise, ddb doesn't even stop the first time for
fatal traps and external breakpoints like the one in kdb_enter().