gril: Switch process id to radio user

Switching process id to radio user allows us to connect to rild without
the need to apply a patch for removing the id check in the Android
sources. After connecting to rild, we switch back to root user.

Based on Jolla code (https://github.com/nemomobile-packages/ofono,
commit 87f4764bb3d4af5c65d3fde875dbdc25978cba9d)
Merge pull request #78 from alfonsosanchezbeato/radio-user

gril: Switch process id to radio user
gril: Add parsing of password requests replies
rilmodem: Retrieve available retries from requests

The ril requests for entering password were returning the number of
available retries for some modems. As this is highly device dependent
it was not being properly addressed.
unit: Available password retries tests