ISLAMABAD — A man whose lawyers said he had been tortured into confessing to murder, and who they said was a minor at the time of the crime, was hanged early Tuesday, despite pleas from rights groups in Pakistan and overseas. The case of the man, Shafqat Hussain, had become a cause celebre in Pakistan, where rights groups portrayed it as a stark example of the country’s flawed judicial system as they renewed calls for abolishing the death penalty.