By William Schomberg LONDON (Reuters) – British Prime Minister Boris Johnson said he wanted to resolve a standoff with the European Union over Northern Ireland’s post-Brexit trade rules, but he kept open the option of unilateral action that the EU says could start a trade war. Johnson was due to travel to Belfast on Monday to urge local political leaders to form a new power-sharing government, a key institution under the 1998 Good Friday peace agreement. After elections this month, pro-British unionists refused to join a new administration because of their opposition to the Northern Ireland Pr…