RFR: 8234262: Unmask SIGQUIT in a child process

[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.

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Commit messages:
 - 8234262: Unmask the SIGQUIT in a child process

Changes: https://git.openjdk.org/jdk/pull/10379/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8234262
  Stats: 65 lines in 2 files changed: 63 ins; 1 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