RFR: 8279488: ProcessBuilder inherits contextClassLoader when spawning a process reaper thread

[Roger Riggs]

The thread factory used to create the process reaper threads unnecessarily inherits the callers thread context classloader.
The result is retention of the class loader.

The thread factory used for the pool of process reaper threads is modified to use an InnocuousThread with a given stacksize.
The test verifies that the process reaper threads have a null context classloader.

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

Commit messages:
 - Remove unnecessary compilation option
 - 8279488: ProcessBuilder inherits contextClassLoader when spawning a process reaper thread

Changes: https://git.openjdk.java.net/jdk/pull/7131/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7131&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279488
  Stats: 103 lines in 3 files changed: 85 ins; 6 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7131.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7131/head:pull/7131

PR: https://git.openjdk.java.net/jdk/pull/7131