RFR: 8234262: Unmask SIGQUIT in a child process [v2]

[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:

  Test both with and without -Xrs

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10379/files
  - new: https://git.openjdk.org/jdk/pull/10379/files/c3b8d950..e0e854bd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 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