Fail if fd is too big and poll() is not available

Fail if fd is too big and poll() is not available

Depending on the process file limit, a file descriptor can be larger
than the capacity of fd_set. There is no portable way to create a
large enough fd_set at run-time. So we just fail if the file descriptor
number is too high and poll() is not available.

Reviewed-by: Jamey Sharp <jamey@minilop.net>
Signed-off-by: Julien Danjou <julien@danjou.info>