RFR: 8307168: Inconsistent validation and handling of --system flag arguments

[Christian Stein]

Please review these changes that fix a bug in `javac`'s validation and handling of `--system` flag arguments.

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Commit messages:
 -  8307168: Inconsistent validation and handling of --system flag arguments

Changes: https://git.openjdk.org/jdk/pull/16264/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16264&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307168
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16264.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16264/head:pull/16264

PR: https://git.openjdk.org/jdk/pull/16264