[crac] RFR: Support extra criu flags from the environment [v2]

[Volker Simonis]

> When testing with different `criu` versions or `criu` configurations it might be useful to be able to modify the default  `criu` command line parameters. This PR introduces the new environment variable `CRAC_CRIU_OPTS` which will be interpreted as a space separated list of `criu` flags to be appended to the hard-coded list of command line parameters. E.g.
> 
> 
> $ CRAC_CRIU_OPTS="-v4 -o resume.log -W /tmp/crac" java -XX:CRaCRestoreFrom=/tmp/crac
> 
> 
> This will set the logging level to 4 (thus overriding the hard-coded logging level of 1 for resuming), redirect the log to the file `resume.log` and change `criu`'s working directory (which will contain the log file) to `/tmp/crac`.

Volker Simonis has updated the pull request incrementally with one additional commit since the last revision:

  Drop surplus arguments from CRAC_CRIU_OPTS and issue a warning

-------------

Changes:
  - all: https://git.openjdk.org/crac/pull/26/files
  - new: https://git.openjdk.org/crac/pull/26/files/35412641..df71ae8d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=26&range=01
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=26&range=00-01

  Stats: 18 lines in 1 file changed: 10 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/crac/pull/26.diff
  Fetch: git fetch https://git.openjdk.org/crac pull/26/head:pull/26

PR: https://git.openjdk.org/crac/pull/26