ISTANBUL   South Korea proposed to the United States to jointly manage parts of the southern half of the heavily fortified Demilitarized Zone separating the two Koreas, local media reported on Thursday. The proposal was put forward by South Korea’s Defense Ministry as Seoul seeks to secure control over civilian access to the 250-kilometer-long (155 miles), 4 km-wide (2.4 miles) DMZ, Yonhap News reported, citing a source. At present, the US-led UN Command administers the military buffer zone as