compat/mingw: Support a timeout in the poll emulation if no fds are given

compat/mingw: Support a timeout in the poll emulation if no fds are given

Our poll() emulation did not support the timeout argument. With this patch
we support it for the simple case where poll() does not need to wait on
file descriptors as well because this case amounts to a mere Sleep().

This is needed if the user sets help.autocorrect is set to a positive
value.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>