RFR: 8267666: Add option to jcmd GC.heap_dump to use existing file

[Anton Kozlov]

Please review a small change that adds an option to GC.heap_dump to use an existing file. 

The option is necessary if the target file is a predefined file-like object, like a named pipe. This opens up a lot of possibilities to process a heap dump without storing it to the file system first.

Reviews of the CSR linked to the bug would be appreciated as well.

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Commit messages:
 - Add jcmd GC.head_dump -rewrite

Changes: https://git.openjdk.java.net/jdk/pull/4183/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4183&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267666
  Stats: 12 lines in 6 files changed: 5 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4183.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4183/head:pull/4183

PR: https://git.openjdk.java.net/jdk/pull/4183