I've revised the patch based on the second round of comments from Martin.  I've removed the ifdef test for CO_TYPE and assumed it is always available.  That means fdtype() should be available on all platforms.

I did not change the test as suggested to call s.detach(), I think my test code should work okay.  The compare of 'protocol' has been removed from the test.

The duplicate definition of SO_PASSCRED has been removed.  I didn't add the constants to the documentation as other constants are only documented on a wildcard basis (e.g. SO_*).