when db is not started, msfconsole launches 'msfdb init' instead of 'msfdb start'

[DidierA]

On a system where the db has already been created, but is not started (ie after first initialisation and a restart of the system), launching msfconsole restarts the initialisation process, asking if we want to create a new db.

in start_db() function, $SCRIPTDIR/msfdb init should be replaced by $SCRIPTDIR/msfdb start.