util: rework word parsing and c unescaping code

When parsing words from input files, optionally automatically unescape
the passed strings, controllable via a new flags parameter.

Make use of this in tmpfiles, and port everything else over, too.

This improves parsing quite a bit, since we no longer have to process the
same string multiple times with different calls, where an earlier call
might corrupt the input for a later call.

src/shared/condition.c

@@ -100,7 +100,7 @@ static int condition_test_kernel_command_line(Condition *c) {
                 _cleanup_free_ char *word = NULL;
                 bool found;
 
-                r = unquote_first_word(&p, &word, true);
+                r = unquote_first_word(&p, &word, UNQUOTE_RELAX);
                 if (r < 0)
                         return r;
                 if (r == 0)