ipn/ipnlocal: clean up c2n handling's big switch, add a mux table

Updates #cleanup

Change-Id: I29ec03db91e7831a3a66a63dcf6ff8e3f72ab045
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f