Optimize mail reinjection

- Mark reinjected mails so that Husk can shortcut the processing.
- Fix Cargo.toml to correctly include the patched version of
  sequoia-cert-store and sequoia-keystore.

src/types/defaults.rs

@@ -11,6 +11,9 @@ pub const PID_FILE_LOCATION: &str = "/run/husk/pid";
 pub const FOOTER: &str = "---\nProcessed by Husk at {date}.\n";
 pub const FOOTER_RECIPIENTS: &str = "This mail was also send to {rcpts}.\n";
 
+/// This ID is used when reinjecting an unencrypted mail into the MTA.
+pub const HELO_REINJECT_ID: &str = "husk.reinject";
+
 /// Default logging configuration.
 pub const CONSOLE_LOGGING: &str = "
 [appenders.console]