FIXED: Possible deadlock in concurrent/3.  Now uses new at_exit option for thread_create/3.
FIXED: Possible deadlock in concurrent/3.  Now uses new at_exit option for
thread_create/3.

edechter/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel