[crac] RFR: PID adjustment on checkpoint [v16]

[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 is adjusted only if Java's PID is 1 that means Java is run in a container. To adjust PID manually for a checkpoint'ed process, `-XX:CRaCMinPid=<value>` option should be used along with `CRaCCheckpointTo`. Min `CRaCMinPid` value is 1, max `CRaCMinPid` value is `UINT_MAX`, but it is actually limited by OS's pid_max.
> 
> There are the following possible scenarios for CRaC running in a container:
> 
>     // getpid   CRaCMinPid  |   set_last_pid      fork
>     // ------------------------------------------------
>     //   1         -        |    yes (default)    yes
>     //   1         1        |    no               yes
>     //   1        >1        |    yes              yes
>     //   >1        -        |    no               no
>     //   >1      <=getpid   |    no               no
>     //   >1       getpid<   |    yes              yes

Roman Marchenko has updated the pull request incrementally with one additional commit since the last revision:

  Waiting for the last child while spinning PID

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

Changes:
  - all: https://git.openjdk.org/crac/pull/86/files
  - new: https://git.openjdk.org/crac/pull/86/files/9b32061f..e95cb986

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=86&range=15
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=86&range=14-15

  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 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