[jdk11u-dev] RFR: 8267666: Add option to jcmd GC.heap_dump to use existing file

[Christoph Langer]

Backport of this enhancement. Some more details to follow. CSR includes update releases.

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Commit messages:
 - Backport 7cbb67a3f8adc83a5b51c092a66480d7b22a6bea

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/131/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=131&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267666
  Stats: 35 lines in 9 files changed: 11 ins; 8 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/131.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/131/head:pull/131

PR: https://git.openjdk.java.net/jdk11u-dev/pull/131