Rename some of the internal HSSF sheet lookup methods which deal with external sheet indicies, to make it clear when they return the first sheet name/index, and add support for getting the last one too (will be the same as the first if not a range)

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1613305 13f79535-47bb-0310-9956-ffa450edef68