i386: prepare to convert exceptions to interrupts

There is some macro magic in traps_32.c to construct standard
exception dispatch functions. This patch renames the DO_ERROR-
like macros to DO_TRAP, and introduces new DO_ERROR ones that
conditionally reenable interrupts explicitly, like x86_64.

No code changes.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>