Some changes to prepare for OEM

This commit:
* Changes gril so you need to pass it the location of the rild socket to create a new GRil object
* Removes the unneeded gril "connected" flag
* Implements g_ril_set_disconnect_function()
* Parses properly signal strength for v5 ril interface
* Adds test-mnglength to .gitignore

To test it, start ofono: if the signal strenght indicator is there it validates the change, as you can connect to rild and are able to receive signal strength event.s