RFR: JDK-8176785 Add build support to generate PNG file from .dot file

[Magnus Ihse Bursie]

This bug is the continuation of JDK-8173303, in which Mandy added the 
generation of .dot files for @moduleGraph Javadoc tags.

With this patch, the code is taken to it's completion, and the 
temporarily solutions left in place by JDK-8173303 has been replaced by 
properly integrated solutions.

Summary of changes:
* A new option --enable-full-docs determines if module graphs should be 
created or not. (In the future, even more optional but "difficult" 
documentation work might be added to depend on this.) This is enabled by 
default is all prerequisites are present.
* It is not impossible to get to a state of the documentation with 
broken img links.
* I have re-introduced SetupJavadocGeneration to stop the code 
duplication between JDK and Java SE javadoc builds.
* I have fixed a bug in how the makefiles determines the dependencies of 
modules. This only affected a single module in nashorn, and no nashorn 
build code used these dependencies, so that's why it has gone unnoticed 
until now. I have verified that no other changes in module dependencies 
are introduced by this fix.
* Javadoc source code dependencies are now correct even for transitive 
modules
* Support added for the creation and use in jib of a "graphviz" module.

This code is dependent on JDK-8172312. This webrev is done as a diff 
against the latest published webrev of JDK-8172312. JDK-8172312 is 
reviewed, but is currently awaiting the status for JEP 299 to be changed 
to Targeted before it can be pushed.

This code only affects the build system and documentation. I intend to 
push it to jdk9 using the "noreg-doc" RDP2 exception.

Bug: https://bugs.openjdk.java.net/browse/JDK-8176785
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8176785-full-build-support-for-module-graphs/webrev.01