BRUSSELS — European Union leaders claimed a breakthrough agreement Friday on how to deal with the pressures of migration after all-night talks helped accommodate Italian demands for more help. The EU leaders said the agreement would bolster the bloc’s external borders and improve the solidarity among member nations to ease pressure on point-of-entry nations like Greece and Italy. The plan proposes screening migrants in North Africa for asylum eligibility and setting up control centers within the bloc by nations which would volunteer to have them.