basic/unit-name: allow unit_name_to_instance() to be used to classify units

This could already be done by calling unit_name_is_*(), but if we don't know
if the argument is a valid unit name, it is more convenient to have a single
function which returns the type or possibly an error if the unit name is not
valid.

The values in the enum are sorted "by length". Not really important, but it
seems more natural to me.

src/test/test-unit-name.c

@@ -430,29 +430,31 @@ static void test_unit_name_to_instance(void) {
         int r;
 
         r = unit_name_to_instance("foo@bar.service", &instance);
-        assert_se(r >= 0);
+        assert_se(r == UNIT_NAME_INSTANCE);
         assert_se(streq(instance, "bar"));
         free(instance);
 
         r = unit_name_to_instance("foo@.service", &instance);
-        assert_se(r >= 0);
+        assert_se(r == UNIT_NAME_TEMPLATE);
         assert_se(streq(instance, ""));
         free(instance);
 
         r = unit_name_to_instance("fo0-stUff_b@b.service", &instance);
-        assert_se(r >= 0);
+        assert_se(r == UNIT_NAME_INSTANCE);
         assert_se(streq(instance, "b"));
         free(instance);
 
         r = unit_name_to_instance("foo.service", &instance);
-        assert_se(r == 0);
+        assert_se(r == UNIT_NAME_PLAIN);
         assert_se(!instance);
 
         r = unit_name_to_instance("fooj@unk", &instance);
         assert_se(r < 0);
+        assert_se(!instance);
 
         r = unit_name_to_instance("foo@", &instance);
         assert_se(r < 0);
+        assert_se(!instance);
 }
 
 static void test_unit_name_escape(void) {