RFR: JDK-8234211 allow discoverable javac plugins to be invoked by default

[Jonathan Gibbons]

Please review a small change to javac to allow plugins to opt-in to 
being automatically started by default if not started explicitly.

The CSR for the API change has already been approved.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8234211
CSR: https://bugs.openjdk.java.net/browse/JDK-8234683
Webrev: http://cr.openjdk.java.net/~jjg/8234211/webrev.00/