Auto merge of #546 - alexander255:master, r=alexcrichton

Add support for the `accept4` system call for FreeBSD* & NetBSD*

As outlined in #540.

However since support for this system call seems to be committed, but not yet released in NetBSD & BitRig (is BitRig actually still be developed at all, btw?), I'm not sure whether it wouldn't be a better idea to only add OpenBSD support instead?
What's the official policy on this?