Fix byte-swapped flowinfo and scope_id

[jgallagher]

The From<SockaddrIn6> for net::SocketAddrV6 implementation used from_be() on each of the port/flowinfo/scope_id fields, but the flowinfo and scope_id are not stored in big endian, causing the conversion to produce a SocketAddrV6 with byte-swapped flowinfo and scope_id. I noticed this when running getifaddrs() on my machine and then converting to a standard SocketAddrV6 created an address with a scope ID of 33554432 (hex 0x02000000) for an interface that should have been scope id 2.

I added a basic test to round-trip between a standard SocketAddrV6 and a SockaddrIn6; prior to the change to the From impl, this test failed with

thread 'sys::socket::addr::tests::sockaddr_in6::roundtrip_std_socketaddrv6' panicked at 'assertion failed: `(left == right)`
  left: `[1234:5678:90ab:cdef::1111:2222%67305985]:258`,
 right: `[1234:5678:90ab:cdef::1111:2222%16909060]:258`

where 67305985 == 0x4030201 is the byte-swapped value of 16909060 == 0x1020304, the expected value.

[asomers]

Duplicate of #1964