By Samia Nakhoul BEIRUT, June 18 (Reuters) - The U.S.-Iran agreement — the first signed by an American and an Iranian president since Iran's 1979 Islamic Revolution — is being hailed by its backers as the deal of the century. But for Tehran's adversaries across the Middle East — from Israel to Gulf states and factions in Lebanon — it looks more like the curse of the century: an accord that could leave Iran more secure, more legitimate and ultimately more influential.