BELGRADE, June 13 (Reuters) - Serbia on Friday rejected a proposed European Union police mission for Kosovo and accused U.N. Secretary-General Ban Ki-moon of overreaching his powers in its former province. Days before Kosovo's constitution comes into effect, Ban put forward a proposal to "reconfigure" the activities of the U.N. Mission in Kosovo, or UNMIK, and allow the EU to pursue a police mission there. "Ban Ki-moon overstepped his powers. The Security Council is the only relevant authority to decide whether the UNMIK mission should be reconfigured," Slobodan Samardzic, Serbia's minister for Kosovo, told reporters. The United Nations has administered...