RFR: JDK-8172562 Changing log level on Javadoc causes total rebuild

[Magnus Ihse Bursie]

This patch makes debugging Javadoc builds a bit easier. It makes sure 
debug flags are not stored in vardeps files, and it also sets the 
-verbose flag on LOG=trace.

Bug: https://bugs.openjdk.java.net/browse/JDK-8172562
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8172562-javadoc-debuggability/webrev.01

/Magnus