Fix to python API extractor and API doc generation

Fix to python API extractor and API doc generation

This fixes a number of issues most of them raised by Eric Blake on the
generated documentation output:
   - parsing of "long long int" and similar
   - add parsing of unions within a struct
   - remove spurious " * " fron comments on structure fields and enums
   - fix concatenation of base type and name in arrays
   - extend XSLT to cope with union in structs

* docs/apibuild.py: fix and extend API extraction tool
* docs/newapi.xsl: extend the stylesheets to cope with union in
  public structures