fix(rosenpass): Integrate signal handlers with mio

With this commit, rosenpass uses a signal handler based on the signal-hook-mio crate.

Even though, in this commit, no rosenpass-rp code is touched, this also
fixes the signal handling in rosenpass-rp. The way rosenpass is
integrated in rp is a bit of a hack – it just directly embeds
rosenpass in the same process (though on a dedicated thread). For this
reason, rp now just inherits rosenpass' signal handlers. The
rosenpass event_loop() will terminate. The main loop of `rp` just spends
most of the time waiting for rosenpass itself to finish, and exits when
it finishes.

Unfortunately, this means we are not using signalfd(2)[^0]; the
signal-hook-mio crate appears to use a pipe-based mechanism to deliver
events to mio instead.

This may not be such a bad thing, as signalfd has some severe drawbacks
with respect to subprocesses and masked signals[^1].

Fixes: #358 (https://github.com/rosenpass/rosenpass/issues/385)
Fixes: #522 (https://github.com/rosenpass/rosenpass/issues/522)
Fixes: #678 (https://github.com/rosenpass/rosenpass/pull/678)

[^0]: https://unixism.net/2021/02/making-signals-less-painful-under-linux/
[^1]: https://ldpreload.com/blog/signalfd-is-useless?reposted-on-request

supply-chain/config.toml

@@ -630,7 +630,11 @@ version = "3.2.0"
 criteria = "safe-to-run"
 
 [[exemptions.signal-hook]]
-version = "0.3.17"
+version = "0.3.18"
+criteria = "safe-to-deploy"
+
+[[exemptions.signal-hook-mio]]
+version = "0.2.4"
 criteria = "safe-to-deploy"
 
 [[exemptions.signal-hook-registry]]