Without ROOM_PASSWORD the guest password defaults to empty, and a client
sending a blank password is granted read+write access (can post). Every
other board's room_server env sets ROOM_PASSWORD; the ThinkNode M6 and M1
were the only two shipping an open room. Default them to "hello" to match
the rest of the tree; operators can still change it at runtime with
'set guest.password'.
Jody Bentley