util: rework word parsing and c unescaping code

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.