[crac] RFR: 8353243: [CRaC] Show all options in engine help

[Timofei Pushkin]

C/R engines are now advised to list all options in `configuration_doc`. If JVM does not let users to control some options it states that in the engine help message.

crexec now documents internal options, such as `image_location` and `exec_location`, in its doc message.

This is how crexec's help looks with this change:

$ java -XX:CRaCEngineOptions=help
crexec - pseudo-CRaC-engine used to relay data from JVM to a "real" engine implemented as an executable (instead of a library). The engine executable is expected to have CRaC-CRIU-like CLI. Support of the configuration options also depends on the engine executable.

Configuration options:
* image_location=<path> (no default) - path to a directory with checkpoint/restore files.
* exec_location=<path> (no default) - path to the engine executable.
* keep_running=<true/false> (default: false) - keep the process running after the checkpoint or kill it.
* direct_map=<true/false> (default: true) - on restore, map process data directly from saved files. This may speedup the restore but the resulting process will not be the same as before the checkpoint.
* args=<string> (default: "") - free space-separated arguments passed directly to the engine executable, e.g. "--arg1 --arg2 --arg3".

Configuration options controlled by the JVM: image_location exec_location

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Commit messages:
 - Show all options in engine help

Changes: https://git.openjdk.org/crac/pull/220/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=220&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353243
  Stats: 86 lines in 5 files changed: 75 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/crac/pull/220.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/220/head:pull/220

PR: https://git.openjdk.org/crac/pull/220