cpuset(3): Move cpuset's parselist function into libutil

In order to allow to add cpuset(2) functionality to more utilities than just
cpuset(1) move the parselist code into libutil

While here, make the code a little more "library" friendly, by returning a range
of various errors so that the consumer can check for them and report appropriate
error message to the users

(One of the planed usage is the jail(8) utility)

Reviewed by: jilles
Differential Revision: https://reviews.freebsd.org/D12873