Changes to rilmodem to make it work with one implementation of the RIL

I adapted the rilmodem driver to work with one implementation of the RIL. Some changes were required, those are proposed here. I am not sure whether those will be compatible with other RIL implementations.

Most notable changes are running ofono as radio user, so that RIL does not require any changes. Other notable change is the GPRS initialization order. Currently, modem is first initialized, GPRS status is requested and then modem is put into online mode. However, this was not compatible with RIL so I changed it so that GPRS status is requested after modem is put online. Other way to solve this problem would be to change the API of the src/gprs.c to allow post_online hook for gprs part of the driver as well.