courier: Let the users configure the mail delivery agent

This patch adds configuration options for the MDA binary and command line
arguments, and changes the (soon to be renamed) procmail courier to make use
of them.

internal/config/config.proto

@@ -18,5 +18,19 @@ message Config {
 	// Address for the monitoring http server.
 	// Default: no monitoring http server.
 	string monitoring_address = 4;
+
+	// Mail delivery agent (MDA, also known as LDA) to use.
+	// This should point to the binary to use to deliver email to local users.
+	// The content of the email will be passed via stdin.
+	// If it exits unsuccessfully, we assume the mail was not delivered.
+	// Default: "procmail".
+	string mail_delivery_agent_bin = 5;
+
+	// Command line arguments for the mail delivery agent. One per argument.
+	// Some replacements will be done:
+	//  - "%user%"   -> local user (anything before the @)
+	//  - "%domain%" -> domain (anything after the @)
+	// Default: "-d", "%user"  (adequate for procmail)
+	repeated string mail_delivery_agent_args = 6;
 }