event: replace `evutil_make_socket_nonblocking()` by an idempotent version

Once a file descriptor is locked with `BIOCLOCK`, it is not possible
to make it non-blocking. Therefore, we need to make it non-blocking
before locking it. Unfortunately, `evutil_make_socket_nonblocking()`
does not check the current value of the flag to see if there is
anything to do and we get a spurious warning. Just provide our version
of this function for this purpose.