cmfs-tools: add cmfs_read_group_desc() in libcmfs

This patch add cmfs_read_group_desc() int libcmfs.
cmfs_read_group_desc() is copied and modified from
ocfs2_read_group_desc() from ocfs2-tools/libocfs2. It read group desc
block which is indecated by blkno, and check group desc magic, if
matches then swap it into cpu byte order.