dhcp4: accept dns-domain list compatibility abuse

The dhcp dns-domain/domain-name option [15] is defined to contain
one single domain name and the dns-search option [119] a list.
Accept a domain name list in the dns-domain option from servers
sending a list for compatibility with attic dhcp clients and log
this abuse as debug. Further, fixed to not require a dns server
list to accept a domain name / search list (bnc#862130).
dhcp: Log all discard and abuse messages as warning

Log a warning instead of a debug message when discarding
invalid/incorrect options send by dhcp-server (bnc#862130).
Merge pull request #151 from mtomaschewski/dhcp-fixes-1

dhcp4: accept and log dns-domain as list compatibility abuse (bnc#862130)