Make spliting system_search_domains more robust

The search line in /etc/resolv.conf could have
multiple spaces or tabs between domains.
split(' ') will give wrong results in some case,
use split() without argument instead.

e.g.
>>> 'domain.tld	cluster.tld '.split(' ')
['domain.tld\tcluster.tld', '']
>>> 'domain.tld cluster.tld '.split()
['domain.tld', 'cluster.tld']