1.  remove impossible test from interio.
2.  remove assert from catio; correct fencepost errors.
3.  remove redundant waserror/error from io.
4.  remove impossible error recovery from mconfig.  memory leaks
     not addressed.
5.  rewrite setdsize so that mirrors, interleavs with a zero-sized inner
     devs don't get confused.  also, interleaved devices are now sized as
     ndev*min(isize) not sum(isize). isize is block-aligned, of course.

undone:  i would like to remove the retries from mirror.  i also
think that mirror should stop on first error.