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.

Cargo.lock

@@ -230,6 +230,16 @@ dependencies = [
  "pkg-config",
 ]
 
+[[package]]
+name = "libudev-sys"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c8469b4a23b962c1396b9b451dda50ef5b283e8dd309d69033475fa9b334324"
+dependencies = [
+ "libc",
+ "pkg-config",
+]
+
 [[package]]
 name = "lock_api"
 version = "0.4.1"
@@ -269,6 +279,18 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "nix"
+version = "0.19.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "85db2feff6bf70ebc3a4793191517d5f0331100a2f10f9bf93b5e5214f32b7b7"
+dependencies = [
+ "bitflags 1.2.1",
+ "cc",
+ "cfg-if 0.1.10",
+ "libc",
+]
+
 [[package]]
 name = "notify-rust"
 version = "4.0.0"
@@ -451,9 +473,11 @@ dependencies = [
  "evdev-rs",
  "hidapi",
  "lazy_static",
+ "nix",
  "notify-rust",
  "parking_lot",
  "signal-hook",
+ "udev",
 ]
 
 [[package]]
@@ -487,6 +511,16 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "udev"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "048df778e99eea028c08cca7853b9b521df6948b59bb29ab8bb737c057f58e6d"
+dependencies = [
+ "libc",
+ "libudev-sys",
+]
+
 [[package]]
 name = "unicode-xid"
 version = "0.0.4"