test: x86: check that illegal opcode (#UD) is reported
exception: report x86 #UD as INT $6
space: pass utcb to sys_unmap()

As the comment in <ukernel/space.h> says, it's accessed by the wrapper, so
doing this avoids a call to thread_get_utcb() (and removes a TODO from
exception.c).
test: x86: add tests for KDB entry and non-KDB int3
exception: fix KDB-entering int3

There's a special case for the test suite, where debug entry strings starting
with "mung " and ending with " do nothing" will simply return to userspace.
This exists so that testbench can invoke KDB without going into an interactive
mode; following testing against L4Ka::Pistachio (where such a feature doesn't
exist), that test should be skipped unless KIP identifies the kernel as mung.
exception: use space_memcpy_from() in handle_kdb_enter()

This is not only nicer, but removes about 26 lines of source too.
exception: split return_from_ud() off, use it for I/O faults & #UD handler

Clears up another FIXME.