Masked soldiers deployed on domestic soil and an air of conspiratorial paranoia give Adam Wingard’s not-quite-grindhouse siege film “Onslaught” a contemporary pulse, even if this lean, B-movie thriller doesn’t deliver much more than a flicker of its throwback promise. Wingard, though, does seem freshly liberated in this return to his low-budget thriller roots (“You’re Next,” “The Guest”). He has since spent the last decade in more IP-managed releases: the 2016 “Blair Witch” sequel, a Netflix adaptation of “Death Note” and a pair of “Godzilla vs. Kong” movies. But the 92-minute “Onslaught,” written by Wingard and his regular collaborator Simon Barrett,...