RFR: JDK-8141261 Clean up building of demos

[Magnus Ihse Bursie]

The handling of demos has not been up to the same quality as the rest of 
the build. Worst of all is the use of non-named argument macros, which 
makes the macro calls very hard to decipher.

The driver behind this fix is that I need to understand properly this 
code to be able to clean up flag handling. But when addressing that, I 
used to chance to also give CompileDemos some well-needed TLC. For 
instance, I have also fixed so:
* "make clean-demos" clean demos.
* Moved closed logic out of open makefiles
* Long specialized recipes are now using simpler, more integrated approaches

Bug: https://bugs.openjdk.java.net/browse/JDK-8141261
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8141261-clean-up-demos/webrev.01

/Magnus