After being forced to stand on the sidelines for eight decades due to state control of the oil industry, private Mexican oil and gas firms came roaring back Tuesday by winning nearly all of the contracts in a government auction of onshore fields thanks to a recent overhaul of the energy industry to embrace private capital. Upstart local oil firms, some brand new and others emerging from the oil-services sector, won 20 of the 25 contracts up for bid in the third and final auction of exploratory blocks and discovered fields this year as part of the remake of a sector to create competition to former monopoly Petróleos Mexicanos.