DDMsgReader can be called directly as a loadable module

DDMsgReader can now be called directly as a loadable module by Synchronet (though requires
the included ddmr_lm.js if DDMsgReader.js is not in sbbs/exec or sbbs/mods).  Also, refactored
DDMsgReader to use attr_conv.js and removed the attribute conversion functions from this script.
Also added a parameter to convertAttrsToSyncPerSysCfg() in attr_conv.js to specify whether or not
to convert ANSI codes to Synchronet attribute codes.