[crac] RFR: Report checkpoint processing to jcmd

[Ilarion Nakonechnyy]

pass output stream from diagnosticCommand.cpp through java code into os_linux.cpp::VM_crac::doit()

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Commit messages:
 - whitespaces
 - Properly pass return code to jcmd peer,
 - Check the opened socket, if it is UNIX socked from jcmd.
 - Implementation of reporting

Changes: https://git.openjdk.org/crac/pull/10/files
 Webrev: https://webrevs.openjdk.java.net/?repo=crac&pr=10&range=00
  Stats: 273 lines in 8 files changed: 216 ins; 17 del; 40 mod
  Patch: https://git.openjdk.org/crac/pull/10.diff
  Fetch: git fetch https://git.openjdk.org/crac pull/10/head:pull/10

PR: https://git.openjdk.org/crac/pull/10