This commit prepares the use of device-specific configuration files.
These files should be named after the device-tree `compatible` string
with the `toml` extension.
`eg25-manager` will search for config files in
`<prefix>/etc/eg25-manager` first (or `/etc/eg25-manager` if prefix is
`/usr`), then in `<prefix>/share/eg25-manager`.
Most of the modem issues follow a (incomplete) USB device reset. Instead
of relying solely on the existing timer, this patch adds a udev monitor
which resets the modem as soon as its associated USB device is reset,
which greatly improves recovery time.
