taskset: fix cpuset list parser

taskset: fix cpuset list parser

taskset hangs when executed with badly formatted cpuset list, for
example:

	 $ taskset -c 0--1 true

The current cpuset list parser is pretty weak as based on scanf()
without strings verification ("-1" as input for "%u" returns
unexpected number). It seems faster and better to use strtoul() and
isdigit().

Addresses: https://github.com/karelzak/util-linux/issues/77
Signed-off-by: Karel Zak <kzak@redhat.com>