resolved: don't claim DnsQuestion have to have the same names

resolved: don't claim DnsQuestion have to have the same names

Wen DnsQuestion objects are used for DnsQuery objects all contained keys
have to share the same name, but otherwise they generally don't have to,
and this can actually happen in real-life because DnsPacket objects for
mDNS use DnsQuestion for the question section.

Hence, rename:

dns_question_is_valid() to dns_question_is_valid_for_query(), since the
name uniqueness check it does is only relevant when used for a query.

Similar, rename dns_question_name() to dns_question_first_name(),
to be more accurate, as this difference matters if we keys don#t have to
share the same name.