Fix behaviour of `\prop_map_tokens:Nn`

There were actually a few issues here:
 - Switch from `\q__prop` to `\s__prop`
 - Missing `\s__prop` in lead-off code
 - Test for termination was incorrect

As now set up, the code works even if the first token in the key part is
`\q_recursion_tail`, as the detokenization of keys means this gives the
correct result. Note that `\__quark_if_recursion_tail_break:nN` gives the
wrong outcome here.



git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4466 de43f980-851b-0410-b2f7-c40aca1f87e0