Add script to parse makefile include graph.

[jonathan]

Summary

This script parses .include and other interesting lines in BSD makefiles
and outputs the result as a GraphViz .dot graph. This allows us to find,
e.g., where in the make system we have defined both CC and LOCALBASE
or both CTFCONVERT_CMD and custom suffix rules.

[jonathan]

• Generalize the metadata we can look for.
• Don't output None attributes.
• Normalize paths.
• Give re-definitions a different shape from definitions.
• Add more filename sanitizing.
• Add flags to filter singletons and unconnected components.
• Print the total number of files being parsed.