YANGON, Myanmar — Security forces in Myanmar opened fire and made mass arrests Sunday as they sought to break up protests against the military’s seizure of power, and a U.N. human rights official said it had “credible information” that 18 people were killed and 30 were wounded. That would be the highest single-day death toll among protesters who are demanding that the elected government of Aung San Suu Kyi be restored to power after being ousted by a Feb.