Added unload/1.  Kept private to ifprolog to only map system:unload/1.
Added unload/1.  Kept private to ifprolog to only map system:unload/1.

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