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/config.rs

@@ -9,7 +9,9 @@ use toml;
 use log4rs;
 
 use crate::types::errors::HuskError;
-use crate::types::defaults::{CONSOLE_LOGGING, SUBJECT_REPLACEMENT};
+use crate::types::defaults::CONSOLE_LOGGING;
+use crate::types::defaults::PID_FILE_LOCATION;
+use crate::types::defaults::SUBJECT_REPLACEMENT;
 
 #[derive(Deserialize, Debug, Clone)]
 pub struct HuskConfig {
@@ -17,6 +19,7 @@ pub struct HuskConfig {
     pub sequoia_home: String,
     pub keyservers: Option<Vec<String>>,
     pub subject_replacement: Option<String>,
+    pub pid_file: Option<String>,
     logging: Option<toml::Value>,
 }
 
@@ -76,6 +79,13 @@ impl HuskConfig {
             SUBJECT_REPLACEMENT.to_string()
         }
     }
+    pub fn get_pid_file(&self) -> String {
+        if let Some(pid_file) = &self.pid_file {
+            pid_file.clone()
+        } else {
+            PID_FILE_LOCATION.to_string()
+        }
+    }
 }
 
 pub type HuskConfigContainer = Arc<Mutex<HuskConfig>>;
@@ -101,7 +111,8 @@ mod tests {
             sequoia_home: "".into(),
             keyservers: None,
             subject_replacement: None,
-            logging: None
+            logging: None,
+            pid_file: None,
         };
 
         assert_eq!(config.get_subject_replacement(), defaults::SUBJECT_REPLACEMENT);