Bug #3048 fix: "commio_has_callback(fd, IOCB_READ, ccb)" assertion;

Bug #3048 fix: "commio_has_callback(fd, IOCB_READ, ccb)" assertion;
may also be applicable to a similar IOCB_WITE assertion.

When we start closing a descriptor, we call commio_finish_callback() to remove
I/O callbacks. If this is not done from commHandleRead or commHandleWrite,
then select(2) structures may still have our descriptor registration and will
call Comm back to read or write before the descriptor is closed for good. This
will trigger a commio_has_callback() assertion.

Based on lp 3p1-rock branch, r9606.