devnum-util: define helper macros for formatting devnum major/minor pairs

And port some parts over.

src/test/test-devnum-util.c

@@ -106,4 +106,19 @@ TEST(device_path_make_canonical) {
         }
 }
 
+static void test_devnum_format_str_one(dev_t devnum, const char *s) {
+        dev_t x;
+
+        assert_se(streq(FORMAT_DEVNUM(devnum), s));
+        assert_se(parse_devnum(s, &x) >= 0);
+        assert_se(x == devnum);
+}
+
+TEST(devnum_format_str) {
+        test_devnum_format_str_one(makedev(0, 0), "0:0");
+        test_devnum_format_str_one(makedev(1, 2), "1:2");
+        test_devnum_format_str_one(makedev(99, 100), "99:100");
+        test_devnum_format_str_one(makedev(4095, 1048575), "4095:1048575");
+}
+
 DEFINE_TEST_MAIN(LOG_INFO);