This doesn't change any behaviour for now, other than maybe running a
full netcheck more often. The intent is to start gathering data on
captive portals, and additionally, seeing this in the 'tailscale
netcheck' command should provide a bit of additional information to
users.
Updates #1634
Change-Id: I6ba08f9c584dc0200619fa97f9fde1a319f25c76
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>