MM: Autoprocess SMS WAPs

Right now, mmsd does not autoprocess SMS WAPs. This was originally due to the fact that Chatty/purple-mm-sms processes SMS WAPs and hands them off to mmsd.

However, it may be a good idea to have mmsd have the functionality to autoprocess SMS WAPs. In this way, mmsd/modem-manager can act as a "true" abstraction layer to get and process MMSes, and not need the assistance of another program to process MMS.

This will be a user configurable setting (defaulting to "Off").