Support multiple inheritance in cluster hierarchy.
Further improve test case/example.
Round corners of rectangular clusters in debug output.
Revert an unintended change.
Catch invalid node IDs given in cluster hierarchy.

Also improves the efficiency of code that counts node presence in
clusters to only walk the hierarchy once.
Fix a bug with multiple-inheritance clusters.

This fixes a bug where non-overlap constraints might not be generated
between some nodes and clusters if their parent cluster had multiple
overlapping clusters.
Added pkg-config files for libcola, libavoid, libvpsc and libtopology.
Fixed library name typo in libtopology.pc