Garbage collection to side-step sparse file descriptor problem.

Garbage collection to side-step sparse file descriptor problem.
(io_select, io_flush, io_iopfromfd): Unused, remove.
(io_open): Allow more than one open for the same file descriptor.
The check is the last remaining use of io_list[], and it's
unimportant.
(io_list, niop, io_init): io_list[] no longer serves any purpose,
remove.  Sparse allocation of file descriptors (deplorable misfeature
of Winsock, at least some versions) is no longer a problem.
(getfdtablesize): Unused, remove.  Was broken for Windows anyway.