* wgengine/magicsock: implement conn.MultiSocketBind
Bind a group of UDP sockets sharing one local port per address family. A
flow index is plumbed through the send path to pick which socket a batch
leaves from.
Group size is fixed at Options.UDPSockets for the life of the Conn. In
case rebind comes short, the group is padded out with conns that block
until closed.
No caller sets Options.UDPSockets yet, every group holds a single socket
and this is inert.
Additionally, add test coverage for the now exercised net/sockopts paths.
Updates tailscale/corp#48168
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>
Change-Id: I253646c753fa46e2efd0ad2158d24c416a6a6964
Alex Valiushko
ea3b9f2472