quic: determine local address for connections

Properly handling path migration requires QUIC connections to know
both the address they are sending from and the address they are
sending to.

Determine the local address for a path by creating a temporary UDP
socket bound to the path; this sends no packets but lets us see
what address the kernel thinks should be used on the path.
(When using a non-*net.UDPConn connection, we don't attempt to do
this on the theory that our PacketConn likely doesn't use the real
network.)

Update fake-network tests to use valid source/destination addresses
for all datagrams.

Change-Id: Ia89f7daf479b10df4c1d23f3e64ad3726a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/832365
Reviewed-by: Nicholas Husin <nsh@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>