RFR: 8261401: Add sanity check for UseSHM large pages similar to the one used with hugetlb large pages [v3]

[Stefan Johansson]

On Thu, 11 Feb 2021 08:38:24 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Only warn if UseLargePages was explicitly set.
>
> Looks good now. Thank you!

Thanks for the reviews @tstuefe and @tschatzl 

FYI, I just filed three more enhancements:
[JDK-8261894: Remove support for UseSHM](https://bugs.openjdk.java.net/browse/JDK-8261894)
[JDK-8261896: Add support for multiple page sizes for UseSHM](https://bugs.openjdk.java.net/browse/JDK-8261896)
[JDK-8261899: Improve warning for UseSHM failures](https://bugs.openjdk.java.net/browse/JDK-8261899)

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PR: https://git.openjdk.java.net/jdk/pull/2488