Open the chat when no command is given

onechat with no command opens the terminal chat, as onechat chat does,
because the chat is the command of daily use once the daemon runs as a
service. A start of the daemon stays explicit, so a second daemon does
not start by accident. onechat help, --help, and -h print the commands,
and an unknown command prints the error and the same help. The install
check runs onechat help in place of the bare command.