This creates a new /api/up endpoint which is exposed in the login
client, and is solely focused on logging in. Login has been removed from
the nodeUpdate endpoint.
This also adds support in the LoginClientView for a stopped node that
just needs to reconnect, but not necessarily reauthenticate. This
follows the same pattern in `tailscale up` of just setting the
WantRunning user pref.
Updates tailscale/corp#14335
Signed-off-by: Will Norris <will@tailscale.com>
