RFR(S) 8046715: Add a way to verify an extended set of command line options

[Jesper Wilhelmsson]

Hi,

As stated in JDK-8046715 [1] I would like to add a call in the argument 
verification to allow for verification and other initialization of extended code 
- that is code not present in the OpenJDK.

Obviously this is a change I need to do in preparation for some Oracle-internal 
code I want to add, but I hope that the hook to initialize extended code will be 
useful to others outside of Oracle as well.

[1] Bug: https://bugs.openjdk.java.net/browse/JDK-8046715
[2] webrev: http://cr.openjdk.java.net/~jwilhelm/8046715/webrev/

Thanks!
/Jesper