8353243: [CRaC] Show all options in engine help

[TimPushkin]

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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Progress

• [x] Change must not contain extraneous whitespace

Issue

• JDK-8353243: [CRaC] Show all options in engine help (Enhancement - P3)

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