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/sysusers/sysusers.c

@@ -1384,7 +1384,7 @@ static int parse_line(const char *fname, unsigned line, const char *buffer) {
 
         /* Parse columns */
         p = buffer;
-        r = unquote_many_words(&p, &action, &name, &id, &description, &home, NULL);
+        r = unquote_many_words(&p, 0, &action, &name, &id, &description, &home, NULL);
         if (r < 0) {
                 log_error("[%s:%u] Syntax error.", fname, line);
                 return r;