Use start.sh to start Pro via msfupdate command

This fixes an issue when using `msfupdate` in an apt environment when Postgres is not started. This changes `msfupdate` to use `/opt/metasploit/scripts/start.sh` instead of `/etc/init.d/metasploit start`. The `start.sh` script (installed with Community/Pro apt installs) takes care of starting dependencies.