* Natgas rig count at lowest since September 2009 * Gas-directed rig count slips for seventh week * Horizontal rig count gains for first time in 4 weeks * Oil rigs slip from near 25-year high By Joe Silha NEW YORK, Feb 24 (Reuters) - The number of rigs drilling for natural gas in the United States fell for the seventh straight week this week as producers continued a slow dry gas drilling operations in the face of low prices. The gas-directed rig count slipped by six t