Instead of using a predefined set of string values for 'ip-type' in
Modem.CreateBearer() and Simple.Connect(), we'll use an enumeration. The
implementation will then need to convert the requested IP family type to e.g.
the correct PDP type in 3GPP modems.
This change also consolidates the use of enums in dictionary properties when
possible to do so, as with the Rm Protocol.
Modems which end up being found unusable (e.g. no SIM, fatal SIM error, no
capabilities) will be exposed in DBus, but just with the Modem interface and
in a FAILED state which allows no actions.
We need to define a state to be used while the modem is being initialized, so
that we forbid any operation on the modem on already exported interfaces, while
there are interfaces pending to get exported.
This Initializing state will also cover the state between having the SIM
unlocked (which launches re-initialization) and being completely initialized.
We don't want to handle bands as flags, in order to avoid the need of 64-bits
for the enum. This change implies that setting allowed bands will be done by
giving an array of uint32 values, signature "au".
