modemmanager: replace modem available check on start with the new monitore service

modemmanager: replace modem available check on start with the new monitore service

Before this change, the status of the sysfs paths from the kernel events
was cached with a cache file. This is necessary to mark configured modems
as available for the netifd.

Using the new monitor service via the mmcli command 'mmcli -M' simplifies
the whole process. There is no need to start sub shells in the background
anymore that monitors whether the modem has already been added to the
ModemManager.

For this purpose, a new service was added that reacts on add and remove
events for modems in the ModemManager and, if necessary, marks the logical
netifd interface as available.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>