WASHINGTON — While Congress mulls how to curtail the NSA’s collection of Americans’ telephone records, impatient civil liberties groups are looking to legal challenges already underway in the courts to limit government surveillance powers. Three appeals courts are hearing lawsuits against the bulk phone records program, creating the potential for an eventual U.S. Supreme Court review. Judges in lower courts, meanwhile, are grappling with the admissibility in terror prosecutions of evidence gained …