feat: move to async POSIX signal handler

Instead of trying to handle signals for every step of the `cursive`
event loop, move the signal handling into its own asynchronous task and
send callbacks to `cursive` when a signal arrives.

Cargo.lock

@@ -1985,6 +1985,7 @@ dependencies = [
  "serde_cbor",
  "serde_json",
  "signal-hook",
+ "signal-hook-tokio",
  "strum",
  "strum_macros",
  "tokio",
@@ -3267,6 +3268,18 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "signal-hook-tokio"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "213241f76fb1e37e27de3b6aa1b068a2c333233b59cca6634f634b80a27ecf1e"
+dependencies = [
+ "futures-core",
+ "libc",
+ "signal-hook",
+ "tokio",
+]
+
 [[package]]
 name = "slab"
 version = "0.4.9"