RFR: 8034944: (process) Improve subprocess handling on Solaris

[Rob McKenna]

Hi folks,

I'd like to push this change into JDK8 & 7. There is other work going on 
in 9 which will supersede this fix so there is likely no point in 
pushing to that release.

This is backport of the threadpool specific changes from 
https://bugs.openjdk.java.net/browse/JDK-6944584 along with some 
cosmetic changes in order to keep the code somewhat similar.

In a nutshell a new process reaper thread was spawned for every Process 
created by the JDK. This fix runs these reaper threads in a thread pool 
to save on thread creation when creating a lot of new processes.

http://cr.openjdk.java.net/~robm/8034944/webrev.01/

     -Rob