RFR: 8296796: Provide clean, platform-agnostic interface to C-heap trimming [v2]

[Thomas Stuefe]

> This is a breakout from [JDK-8293114](https://bugs.openjdk.org/browse/JDK-8293114), which is starved for reviews. So I attempt to break up that fix into smaller units which are hopefully easier to review separately.
> 
> We can trim the C-heap manually using jcmd since [JDK-8268893](https://bugs.openjdk.org/browse/JDK-8268893). This patch reshapes this code, cleaning it up in an OS-agnostic way. That will allow us to add implementions for other platforms (I have this on my list for AIX at least) and make review of 8293114 easier.

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  Feedback David

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11089/files
  - new: https://git.openjdk.org/jdk/pull/11089/files/3b93fb89..1a642e0d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11089&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11089&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11089.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11089/head:pull/11089

PR: https://git.openjdk.org/jdk/pull/11089