Listen for the TTY port using udev

Closes #1

See merge request Librem5/haegtesse!3

To stop the main.c becoming even more unwieldy, most of the code is
moved into some new high-level objects, HaegPort and HaegAudio.  We
also introduce a new object, HaegPortMonitor, to encapsulate the udev
interaction.

Closes #1

Error handling

See merge request Librem5/haegtesse!2

Instead call a new function, haeg_error, which calls g_log_v with a
level of critical, and then calls exit().

debian/haegtesse.user-service: Spell "daemon" properly

See merge request Librem5/haegtesse!1