Subsystem management updates in the core

[Aleksander Morgado]

Hey all,

I've reworked the core daemon logic related to device subsystems, see
changes and description in gitlab:

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/388

The main purpose is to make it easier to introduce new subsystems
(e.g. rpmsg for qualcomm socs, or mhi for PCIe devices).

Comments welcome!

-- 
Aleksander
https://aleksander.es