feat(tvix/store): fix ctrl-c handling on mount command

This enables the tokio `signal` feature, and registers a ctrl_c signal
handler, which will use the unmount handle to unmount in case a ctrl-c
signal is received.

This avoids having disconnected mountpoints when Ctrl-C'ing a
`tvix-store mount` invocation.

In case the filesystem is unmounted externally (via `umount /path/to/
mountpoint`), the future is waiting for the signal is never resolved and
the task is stopped.

Change-Id: I149f705a6cb50188177f2a6c6a5fcd77218e2a3f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9218
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/Cargo.lock

@@ -2116,6 +2116,15 @@ dependencies = [
  "lazy_static",
 ]
 
+[[package]]
+name = "signal-hook-registry"
+version = "1.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d8229b473baa5980ac72ef434c4415e70c4b5e71b423043adb4ba059f89c99a1"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "slab"
 version = "0.4.8"
@@ -2433,6 +2442,7 @@ dependencies = [
  "mio",
  "num_cpus",
  "pin-project-lite",
+ "signal-hook-registry",
  "socket2",
  "tokio-macros",
  "windows-sys 0.48.0",