mark all fds used for select() as non-blocking

This shouldn't matter much, but there _are_ (rare) situations where
an fd is marked ready for reading, but a subsequent read will block
nevertheless. It's never wrong to use non-blocking fds, so just do it.

This patch also includes a check in para_fd_set() which spots blocking
fds as they are added to the fd sets. This check is commented out
for performance reasons but can easily be activated.