Use cythonized SO_REUSEPORT rather than the unwrapped native one.

[ptribble]

The code already detects (via hasattr) whether SO_REUSEPORT is available, setting has_SO_REUSEPORT, and creates a wrapped SO_REUSEPORT (via getattr). This change ensures it's that wrapped SO_REUSEPORT that's used, rather than the native C version (which may not exist).

Fixes #550