ENH: Kill CLI executable process on Slicer exit.

The use of a MutexLock is required because the process have been
initiated from a thread and they are killed by calling "itksysProcess_Kill()"
from the main thread.

In "ApplyTask()", the MutexLock is Locked/Unlocked at the position:

1) After calling "itksysProcess_Kill()" in "KillProcesses()", the call to
"itksysProcess_WaitForExit()" in "ApplyTask()" leads to a call
"kwsysProcessCleanup". Since calling "kwsysProcessCleanup" frees the memory
(i.e free ForkPIDs, RealWorkingDirectory and also cleanup descriptor)
 and that "kwsysProcess_Kill" expects the ForkPIDs array to be available,
it is required to use a mutex to make sure the call to "kwsysProcess_Kill"
completes before "itksysProcess_WaitForExit" is executed.

2) When calling "itksysProcess_Delete()" and updating the vector of
"Processes" that is accessed from both the main thread and the ApplyTask
thread.

Fixes #3395

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22796 3bd1e089-480b-0410-8dfb-8563597acbee