libutil: Move cpuset(1) domain policy parsing code into libutil

cpuset(1) implements a domainset(9) policy parser that is used to
translate a <policy>:<domain_list> string into a domainset_t mask
and a valid domainset policy. This change moves the domainset parsing
code into a new cpuset.c function - 'domainset_parselist'.

The existing cpuset.c code was refactored into a generalized list
parsing function which is now used to parse both CPU sets and domain
sets. This is the same approach used in cpuset(1).

Reviewed by: markj, ziaee (manpages)
Differential Revision: https://reviews.freebsd.org/D46607