libgfs2: Add abstractions for rgrp tree traversal
    
    Users of libgfs2 are required to include the gfs2-utils-specific
    osi_tree.h in order to make use of the resource groups. With this patch
    we introduce functions which wrap the osi_first and osi_next functions
    so that we can hide the implementation of the resource group tree behind
    the lgfs2_rgrp_t type. Converting users of the osi_tree.h functions to
    the new rgrp APIs will be done later and gradually.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>