db_nextframe/i386: reduce the number of special frame types

This change removes TRAP_INTERRUPT and TRAP_TIMERINT frame types.

Their names are a bit confusing: trap + interrupt, what is that?
The TRAP_TIMERINT name is too specific -- can it only be used for timer
"trap-interrupts"? What is so special about them?

My understanding of the code is that INTERRUPT, TRAP_INTERRUPT and
TRAP_TIMERINT differ only in how an offset from callee's frame pointer to a
trap frame on the stack is calculated. And that depends on a number of
arguments that a special handler passes to a callee (a function with a
normal C calling convention).

So, this change makes that logic explicit and collapses all interrupt frame
types into the INTERRUPT type.

Reviewed by: markj
Discussed with: kib, jhb
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D22303