RFR: 8279488: ProcessBuilder inherits contextClassLoader when spawning a process reaper thread [v2]

[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.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Folded lines to reduce max line length

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7131/files
  - new: https://git.openjdk.java.net/jdk/pull/7131/files/3e64a424..fd3faa9d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7131&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7131&range=00-01

  Stats: 12 lines in 1 file changed: 6 ins; 0 del; 6 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