RFR: 8298638: Cleanup of unix java_md.c for better re-exec handling

[Dmitry Samersoff]

Move "unix" java_md.c cleanup, which is the prerequisites for the fix for JDK-8293806 (PR https://github.com/openjdk/jdk/pull/11538), to a separate PR.

@AlanBateman, @dholmes-ora please, take a look to the changes.

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Commit messages:
 - 8298638: Cleanup of unix java_md.c for better re-exec handling

Changes: https://git.openjdk.org/jdk/pull/11645/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11645&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298638
  Stats: 114 lines in 1 file changed: 37 ins; 46 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/11645.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11645/head:pull/11645

PR: https://git.openjdk.org/jdk/pull/11645