select/cons_res - allocate cores cyclic across sockets

Restore default behavior of allocating cores to jobs on a cyclic basis
across the sockets unless SelectTypeParameters=CR_CORE_DEFAULT_DIST_BLOCK
or user specifies other distribution options.
Reverts commit 7fcdc7e5488b1fa7d4305a85d68138d360ea5cb4
bug 466
Merge branch 'slurm-2.6'