RFR: 8297451: ProcessHandleImpl should assert privilege when modifying reaper thread [v2]

[Ryan Ernst]

> This commit guards thread modifications for the process reaper thread with doPrivileged.

Ryan Ernst has updated the pull request incrementally with two additional commits since the last revision:

 - Merge pull request #1 from ChrisHegarty/reaper_thread_modify
   
   Add privileged helper method and update existing test
 - Add privileged helper method and update existing test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11309/files
  - new: https://git.openjdk.org/jdk/pull/11309/files/c02f3f09..a822cc8e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11309&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11309&range=00-01

  Stats: 31 lines in 2 files changed: 20 ins; 6 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11309.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11309/head:pull/11309

PR: https://git.openjdk.org/jdk/pull/11309