ISTANBUL (Reuters) - Turkish President Tayyip Erdogan won sweeping new executive powers on Monday after his victory in landmark elections that also saw his Islamist-rooted AK Party and its nationalist allies secure a majority in parliament. Erdogan’s main rival, Muharrem Ince of the Republican People’s Party (CHP), conceded defeat but branded the elections “unjust” and said the presidential system that now takes effect was “very dangerous” because it would lead to one-man rule.