WASHINGTON (Reuters) - Lawmakers eager to reduce gun violence in their states are proposing mandatory liability insurance for American firearm owners as a new way to limit deaths and injuries. Provoked by the December 14 massacre of 20 schoolchildren and six adults at a school in Newtown, Connecticut, the legislators hope to harness market forces as another tool for gun control. Proponents argue that operators of vehicles, for example, must have liability insurance, so gun owners should as...