core: new `Initializing' state in the global modem state machine

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 define custom nicks for each error enum, matching the DBus error entry that
we want to have with each of them.

Lets operations specify that they cannot be run currently, but that the caller
may retry later. Of course, it depends on the caller to really honour the
retry-later suggestion.

So that glib-mkenums detects that we're talking about flags.