8226242 : Diagnostic output for posix_spawn failure

[Roger Riggs]

Please review adding diagnostic information to the native code handing 
failures
to exec the jspawnhelper.  Some information about the child process 
status can be included.

Jira:
   diag task: https://bugs.openjdk.java.net/browse/JDK-8226242
   Issue:  https://bugs.openjdk.java.net/browse/JDK-8226192

Webrev:
   http://cr.openjdk.java.net/~rriggs/webrev-spawn-diag-8225192/

Thanks, Roger