This adds a portable way to do a raw LocalAPI request without worrying
about the Unix-vs-macOS-vs-Windows ways of hitting the LocalAPI server.
(It was already possible but tedious with 'tailscale debug local-creds')
Updates tailscale/corp#24690
Change-Id: I0828ca55edaedf0565c8db192c10f24bebb95f1b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>