sprinkle NOTREACHED and FALLTHROUGH

sprinkle NOTREACHED and FALLTHROUGH

Coverity complains bitterly about many switch statements in lots of files.

Many of these are of the form:

    case FOO:
        ...
        goto baz;
    case BAR:
        ....

and something as smart as Coverity shouldn't really be complaining about
a missing 'break' when the last statement of the previous branch is an
unconditional goto/return/continue. But I've shoved in a bunch of
'NOTREACHED' to hopefully shut it up.

Missing 'FALLTHROUGH' comments were more reasonable, and I've added them
where appropriate.

The only confusing one was cx_dup(), where the various CXt_LOOP_ branches
all fell through to the next one, and it took a while to figure out that
those weren't bugs.