fix #17185 - callback execution at exit and -nogui argument fixed

The freeze was due to an extra execution of a priority command (a graphic callback) when Scilab was in exit mode (force quit == true). Because of the exit mode, the tread that unstacks commands was closed, so the callback could not be executed and remained in the store command. The execution of scilab.quit gets interrupted due to the priority of the callback command that makes Scilab wait for a signal that must be sent by the closed thread. This fix check that the command store is well empty before leaving the thread.

Fix -nogui to have the same behavior than -nwni.