Add daemon CLI commands

- Introduce option `--background` for `daemon start` to send the new
  process into the background.
- Implement a `ProcessDescr` which identifies a Husk process. This pays
  attention to the possibility of a rollover of the process ids.
- Implement `daemon status`.
- Implement `daemon stop` which sends `SIG_INT` to a running Husk
  process.

src/types/defaults.rs

@@ -6,6 +6,8 @@ pub const CONFIG_FILE_LOCATION: &str = "/etc/husk/config.toml";
 /// and replaced by `SUBJECT_REPLACEMENT` (unless configured otherwise).
 pub const SUBJECT_REPLACEMENT: &str = "...";
 
+pub const PID_FILE_LOCATION: &str = "/run/husk/pid";
+
 /// Default logging configuration.
 pub const CONSOLE_LOGGING: &str = "
 [appenders.console]