RFR: 8243208: Clean up JVMFlag implementation

[Ioi Lam]

This is the same patch as
[8243208-cleanup-jvmflag-impl.v02](https://cr.openjdk.java.net/~iklam/jdk16/8243208-cleanup-jvmflag-impl.v02/)
published in
[hotspot-dev at openjdk.java.net](https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-September/042863.html).

The rest of the review will continue on GitHub. I will add new commits to respond to comments to the above e-mail.

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Commit messages:
 - 8243208: Clean up JVMFlag implementation

Changes: https://git.openjdk.java.net/jdk/pull/82/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=82&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8243208
  Stats: 2034 lines in 45 files changed: 695 ins; 516 del; 823 mod
  Patch: https://git.openjdk.java.net/jdk/pull/82.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/82/head:pull/82

PR: https://git.openjdk.java.net/jdk/pull/82