Loads up the appropriate "Subs-Db" file and connects to the database

• Determines which file to load based on $db_type
• If $db_type is empty or no "Subs-Db" file exists for that type, defaults to MySQL
• If you're using SSI, checks for separate ssi db username and password, and connects with those if they're set
• Connects with regular username and password if not in SSI or if SSI db connection failed
• Doesn't select the DB if you're in SSI