RFR: 8234262: Unmask SIGQUIT in a child process [v4]

[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:

  Remove unnecessary java.lang imports

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10379/files
  - new: https://git.openjdk.org/jdk/pull/10379/files/dc56e0d8..727b0b2a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10379&range=02-03

  Stats: 3 lines in 1 file changed: 1 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