Don't override ARES_OPT_DOMAINS with the values in resolv.conf

I noticed that since commit 125b1a8619eb27556e093fd9c9adf451e896f012, "domains" and "search" lines in resolv.conf are overriding search domains set by the ARES_OPT_DOMAINS option.

This patch fixes init_by_resolv_conf to get the old behaviour back where ARES_OPT_DOMAINS options take priority, but still makes the last "domain" or "search" resolv.conf entry take priority over the first if the user didn't pass in any ARES_OPT_DOMAINS.