WASHINGTON (Reuters) - Emails between U.S. diplomats in Islamabad and State Department officials in Washington about whether to challenge specific U.S. drone strikes in Pakistan are at the center of a criminal probe involving Hillary Clinton's handling of classified information, the Wall Street Journal reported on Thursday. The 2011 and 2012 emails were sent via the "low side" -government slang for a computer system for unclassified matters - as part of a secret arrangement that gave the State Department more of a voice in whether a CIA drone strike went ahead, according to congressional and law enforcement officials briefed on...