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/userdb/userdb.go

@@ -79,6 +79,13 @@ var (
 	InvalidUsernameErr = errors.New("username contains invalid characters")
 )
 
+func New(fname string) *DB {
+	return &DB{
+		fname: fname,
+		users: map[string]user{},
+	}
+}
+
 // Load the database from the given file.
 // Return the database, a list of warnings (if any), and a fatal error if the
 // database could not be loaded.
@@ -194,7 +201,11 @@ func (db *DB) Write() error {
 			base64.StdEncoding.EncodeToString([]byte(user.password)))
 	}
 
-	return safeio.WriteFile(db.fname, buf.Bytes(), db.finfo.Mode())
+	mode := os.FileMode(0660)
+	if db.finfo != nil {
+		mode = db.finfo.Mode()
+	}
+	return safeio.WriteFile(db.fname, buf.Bytes(), mode)
 }
 
 // Does this user exist in the database?