Removed MAX_FD configure test

It only returns the current max anyway, which is usually
smaller than the max if you are running as root, and can
also be changed at any time at runtime.

Instead, use sysconf(_SC_OPEN_MAX) when available if needed, and as a
last resort a hardcoded value (on Windows), but as a secondary change
all uses except two was removed:

o Stdio.get_all_active_fd - On systems without /dev/fd and
  /proc/self/fd.

o On NT, the size of FD_SET is now set to a hardcoded value (64k).

The rewrite of get_all_active_fd also speeds it up a lot. Not that it
is a frequently used function.