[crac] RFR: PID adjustment on checkpoint [v6]

[Roman Marchenko]

> On restore, there might be PID value conflicts because of small PID values, if it was checkpoint'ed in a container. Therefore, when checkpointing in a container, we need to move PID value for new processes to a particular value to avoid conflicts on restore.
> 
> See https://github.com/CRaC/example-lambda/blob/master/checkpoint.cmd.sh#L8 for example.
> 
> This PR contains implemented functionality similar to the example above, making this work out of the box. By default, if checkpointing, PID value for new processes starts from 128. 
> 
> To set a custom value, `CRAC_MIN_PID` environment variable should be used. 
> Min `CRAC_MIN_PID` value is 1, max `CRAC_MIN_PID` is not implemented currently.

Roman Marchenko has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 13 additional commits since the last revision:

 - Adapting tests
 - Fixing JVM parameter definition
 - Set MaxSpinCount to PID value
 - Merge branch 'crac' into pid-adjustment
 - Removing unnecessary test code
 - Implemented vm option instead of env var
 - Added test for the last pid overflow while spinning
 - Re-worked spin_last_pid procedure
 - Re-worked set_last_pid procedure
 - Replacing fork() with clone() for Linux platform
 - ... and 3 more: https://git.openjdk.org/crac/compare/4ff40b58...67dbf0ed

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

Changes:
  - all: https://git.openjdk.org/crac/pull/86/files
  - new: https://git.openjdk.org/crac/pull/86/files/05afa134..67dbf0ed

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=86&range=05
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=86&range=04-05

  Stats: 1116 lines in 20 files changed: 1074 ins; 8 del; 34 mod
  Patch: https://git.openjdk.org/crac/pull/86.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/86/head:pull/86

PR: https://git.openjdk.org/crac/pull/86