src: make bearer/operator enums/func public

Moved enums for operator_status and packet_bearer
to common.h to avoid duplication in drivers. Also
moved packet_bearer_to_string() from gprs.c to
common.c for the same reason.
build: modify unit_test_gril*_SOURCES

The gril code now uses some utility functions from
src/common.c, so this change fixes the build for
the associated unit tests.
gril: added rilmodem gprs/netreg parcel support

This change adds support for building and parsing
rilmodem gprs and network-registration messages.
rilmodem: re-factor rilmodem gprs and netreg

General re-factoring of rilmodem gprs and
netreg code, including:
 - merging applicable code from nemomobile/ofono [1]
 - moved associated parcel handling code to gril
 - add support for BEARER (LP: 1245954)

[1] https://github.com/nemomobile-packages/ofono