This doesn't yet fully pull it out into a feature/captiveportal package.
This is the usual first step, moving the code to its own files within
the same packages.
Updates #17254
Change-Id: Idfaec839debf7c96f51ca6520ce36ccf2f8eec92
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>