added bm818-watchmodem

To monitor the availability of the bm818 modem in the Librem5 this repo has been re-organized.

The former bm818-tools now builds as bm818-volte and a new package is build from the repo: bm818-watchmodem.

This way more different packages for the bm818 modem could be build from this repo in the future.

The vanishing modem (and thereby not reliable reachability of the Librem5) is a real problem for new users and I'd like to help to put out some package helping to make sure that the user at least is informed about modem problems. 'watchmodem' tries to reset the modem in a way that proved to work on my L5 and according to the kernel issue for others as well.

The way watchmodem informs the user could possibly be improved in ways (by e.g. respecting the feedback settings or using feedback) and the service could be replaced by a udev rule watching the modem.

But first of all I'd love to see this work have its place somewhere and make it in an initial version packaged to users who do need to rely on the connectivity of the Librem5 and experience problems with the modem (like myself ;-) ).