w32: Make npth_eselect actually work.

* w32/npth.c (npth_eselect): Rewrite.

It never worked because the result FDSETs were first cleared and then
used to see which fd to check after the WFMO. The new way better
resembles the way a select is expected to work under Windows (ignoring
the first arg) and will also work if this library is used by
application which have been build with a different FD_SETSIZE.