udp: Windows: abstract the GetSocketOption away too.

Windows is weird, put this stuff in the impl. specific sub file too.
Move more syscall function into their own impl.

Windows is stupid
typo
And add the import
Added GOOS={linux,darwin,windows} to the Travis-CI build matrix
Revert "And add the import"

This reverts commit c00b800150b59349717393c0cef5849c80dfdfa9.
Revert "typo"

This reverts commit 4ae6bac881e05eb163d2c84dcecc62f0604ff4a9.
Revert "Move more syscall function into their own impl."

This reverts commit e571f699d21c9f1a2fa2baa1c50c3cdbdb024769.
Revert "udp: Windows: abstract the GetSocketOption away too."

This reverts commit 907e91be5750a858976be3effc87c908bb5a3885.