492317 Fix Relax-NG validation problems

* relaxng.c xmlregexp.c: a subtle problem when checking for compileable
  content model, if using the same elements in cases of choices. Handled
  by adding a special flag to the regexp compilation to detect
  transitions with different atoms using same strings.
* test/relaxng/492317* result/relaxng/492317*: add the test to the
  regression suite