cpu-set-util: introduce config_parse_cpu_set()

cpu-set-util: introduce config_parse_cpu_set()

Then, make parse_cpu_set() as a tiny wrapper of it.

Note, previously when an invalid CPU range, e.g. "3-0", is specified,
we ignore the range but allocate an empty set. But, with this commit,
now the conf parser simply ignore it without no side effect.
This potentially changes behavior of a system with such invalid setting,
but the change should be favorable for consistency with other parsers.