Add in/out/seen commands to let people voluntarily check in and out of
tracks and other arbitrary locations, to make it easy for others to
find them. Of course this is entirely optional.
It's designed to cope gracefully with people forgetting to check out
of locations they previously checked into.
Change-Id: I0d88a540ad7a333841c208dd7f2a7247897eb238
946d69b140