Test for and catch 'nested' choice keys (fixes #171)

The test here is at point of defintion, rather than use, as it's really
about a defective set up.

The same issue applies to multichoice keys, and as the code is rather
repetitive it's also combined here as much as possible.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5093 de43f980-851b-0410-b2f7-c40aca1f87e0