QIO and catching rendez_sleep()

rendez_sleep() throws an error.  In qbwrite(), this is after the point
where we published the block b to the queue.  If the rendez_sleep() is
aborted, we'd error out and freeb(b), which is still on the queue.

Our use of error() in rendez_sleep() might be different than Plan 9.  We
already have slightly different rendez semantics.  Though many rendez
calls are clearly wrapped in waserrors() in other places.

I looked at some of the other rendez spots, like devmnt, and at least
put some warnings in place for the next time we have potential rendez
issues.
Process create/destroy fixups

If sys_proc_create() failed early, a variety of fragile refcntings and
other bits would be messed up.  Mostly since we want to destroy and we
haven't __proc_readied, but destroy cleans up after both alloc and
ready.