Keep track of when we have a mapSession and use it for discoKey
injection if we have one, if not, fall back to injecting via magicsock
like was done already.
This is WIP, should not be merged as is. It is only here as an
illustration of what a general shape could look like.
Updates #12639
Signed-off-by: Claus Lensbøl <claus@tailscale.com>
a60ef6cee3