cpuset: Use stride in cpulist_create

cpuset: Use stride in cpulist_create

Format cpu lists for `taskset -c`, `lscpu`, etc. using stride.  This
usually produces shorter output.

Now:
% ./taskset -c 0-24:2,25-47:4 ./taskset -pc 0
pid 3937653's current affinity list: 0-24:2,25-45:4

Previously:
% ./taskset -c 0-23:2,24-47:4 ./taskset -pc 0
pid 3933321's current affinity list: 0,2,4,6,8,10,12,14,16,18,20,22,24,28,32,36,40,44

The ranges are constructed greedily; in general it is difficult to
construct a minimal list.
% ./taskset -c 0-63:3,0-63:4 ./taskset -pc 0
pid 3926213's current affinity list: 0,3,4-8:2,9-15:3,16-20:2,21-27:3,28-32:2,33-39:3,40-44:2,45

Signed-off-by: Jesse Rosenstock <jmr@google.com>