RFR (S) round 0 for 8141068 Refactor -XXFlags= code in preparation for removal

[Ron Durbin]

This bug fix consists of a refactoring/cleanup of the -XX:Flags code.

Webrev:
http://cr.openjdk.java.net/~rdurbin/8141068_OCR0_JDK9_webrev


Bug report:
https://bugs.openjdk.java.net/browse/JDK-8141068

This bug fix has been tested on:
   OS:
     Solaris, MAC, Windows, Linux
   Tests:
     Manual unit tests
     JPRT with -testset hotspot(including the SQE current test coverage for this feature.)