By Pavel Polityuk and Aleksandar Vasovic LVIV/KYIV, Ukraine (Reuters) -Fighting stopped about 200,000 people from evacuating the besieged Ukrainian city of Mariupol for a second day in a row on Sunday, as Russian President Vladimir Putin vowed to press ahead with his invasion unless Kyiv surrendered. Most people trapped in the port city are sleeping underground to escape more than six days of near-constant shelling by encircling Russian forces that has cut off food, water, power and heating supplies, according to the Ukrainian authorities. In a phone call with Turkish President Tayyip Erdogan,…