President Clinton has signed into law the Defense Appropriations Bill, which will set up a new dental benefit for reservists. The Defense Department is expected to come up with a plan by Oct. 1 on how to provide dental coverage for more than one million reservists. It guarantees that the reservist would pay no more than $20 a month in premiums for coverage. The benefit was prompted by the military`s experience during the Persian Gulf War in 1991 when hundreds of Reserve and National Guard units were called up for service and found they were not "dental ready." More than one million people will be eligible for the benefit.