RFR: 8234262: Unmask SIGQUIT in a child process [v3]

[Roger Riggs]

> Clear the signal mask of the child when launching with posix_spawn.
> 
> SIGQUIT signals are handled on non-Java Threads by the VM.
> For Java threads the signal mask blocks SIGQUIT.  
> The ProcessBuilder uses posix_spawn on all platforms to create new processes.
> Without a specific request, the child process inherits the signal masks from the parent.

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

  Move clearing of the signal mask to jspawnhelper so the launched
  process gets a clear signal mask.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10379/files
  - new: https://git.openjdk.org/jdk/pull/10379/files/e0e854bd..dc56e0d8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=01-02

  Stats: 20 lines in 2 files changed: 7 ins; 12 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10379.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10379/head:pull/10379

PR: https://git.openjdk.org/jdk/pull/10379