courier: Tidy up the Procmail courier

This patch tidies up the Procmail courier:
 - Move the configuration options to the courier instance, instead of using
   global variables.
 - Implement more useful string replacement options.
 - Use exec.CommandContext for running the command with a timeout.

As a consequence of the first item, the queue now takes the couriers via its
constructor.

internal/config/config.go

@@ -55,7 +55,7 @@ func Load(path string) (*Config, error) {
 	}
 	if len(c.MailDeliveryAgentArgs) == 0 {
 		c.MailDeliveryAgentArgs = append(c.MailDeliveryAgentArgs,
-			"-d", "%user%")
+			"-f", "%from%", "-d", "%to_user%")
 	}
 
 	if c.DataDir == "" {