odhcpd: fix DNS-server and MSR-entries in RA

Similarly to the previously broken address pools, DNS-servers and some
MSRs could be advertised incorrectly as well. This is now fixed.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39739 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcpd: avoid the unspecificed address being advertised as DNS

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39740 3c298f89-4303-0410-b956-a3cf2f4a3e73