Make build.tools.jigsaw.GenGraphs more configurable

GenGraphs tool generates the module graph. It currently supports the configuration via javadoc-graphs.properties. However, make/jdk/src/classes/build/tools/jigsaw/javadoc-graphs.properties only documents two properties. It should be updated to cover all configurable properties.

There are a couple other properties not configurable such as nodesep and node margin. This extends the configuration to allow to set additional properties.