guix-package: Error out when passed a non-option argument.

* guix-package.in (guix-package)[parse-options]: Call `leave' when
  passed a non-option argument.
  Reported by Andreas Enge <andreas@enge.fr>.
* tests/guix-package.sh: Add test.

guix-package.in

@@ -326,7 +326,7 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
                (lambda (opt name arg result)
                  (leave (_ "~A: unrecognized option~%") name))
                (lambda (arg result)
-                 (alist-cons 'argument arg result))
+                 (leave (_ "~A: extraneous argument~%") arg))
                %default-options))
 
   (define (guile-missing?)

tests/guix-package.sh

@@ -139,3 +139,6 @@ fi
 # Failed attempt to roll back.
 if guix-package --bootstrap --roll-back;
 then false; else true; fi
+
+# Extraneous argument.
+! guix-package install foo-bar