We're going to add a second SO_REUSEPORT use in the codebase. Extract
and generalize udprelay mechanism for future reuse. Also address the
Linux flaw of allowing same UID groups to merge if the ephemeral port
selection collides.
Exposes public SetReusePort, RawConn and ControlFunc signatures for
shared use.
Updates tailscale/corp#48168Fixestailscale/corp#49297
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>
Change-Id: Iba33142877513d72123c51cf1bcc33cf6a6a6964