use libudev to handle device disconnect/reconnect

This change has substantially bumped up the daemon's overall robustness,
as the code now ensures that the controller will only start once the
/dev/input/eventXX file is set up, which was causing all sorts of issues
in the past.

Additionally, this change enables the daemon to run as a proper
background task that _doesn't_ constantly die / need to be restarted,
which removes the need for any janky udev "on add" rules, and instead, a
simple systemd user service will suffice.

install/50-surface-dial.rules

@@ -1 +0,0 @@
-ACTION=="add", ATTR{name}=="Surface Dial System Multi Axis", TAG+="systemd", ENV{SYSTEMD_USER_WANTS}="surface-dial.service"

install/surface-dial.service

@@ -1,5 +1,12 @@
-# IMPORTANT: modify the "ExecStart" field to reflect your userid + surface-dial-daemon install dir
+# IMPORTANT: modify the "ExecStart" field to reflect your surface-dial-daemon install dir
+
+[Unit]
+Description=Surface Dial Daemon
 
 [Service]
-# HACK: this service needs to run _after_ the /dev/input/eventXX files have been created
-ExecStart=bash -c 'sleep 1 && /home/danielprilik/.cargo/bin/surface-dial-daemon'
+Type=simple
+StandardOutput=journal
+ExecStart=/home/danielprilik/.cargo/bin/surface-dial-daemon
+
+[Install]
+WantedBy=default.target