Remove blocking struct iop operation

Remove blocking struct iop operation

The blocking I/O option makes no sense in the server, because it
blocks the server process instead of the thread.  In fact, it's been
unused since Empire 2, except for one place, where it was used
incorrectly, and got removed in the previous commit.

Make I/O non-blocking in io_open() unconditionally.  Remove IO_NBLOCK
and io_noblocking().