Make mdns responder case-insensitive.

Make mdns responder case-insensitive.

From [the RFC](https://github.com/openwrt/mdnsd/blob/master/rfc6762.txt#L2532-L2550):

>  The simple rules for case-insensitivity in Unicast DNS [RFC1034]
   [RFC1035] also apply in Multicast DNS; that is to say, in name
   comparisons, the lowercase letters "a" to "z" (0x61 to 0x7A) match
   their uppercase equivalents "A" to "Z" (0x41 to 0x5A).  Hence, if a
   querier issues a query for an address record with the name
   "myprinter.local.", then a responder having an address record with
   the name "MyPrinter.local." should issue a response.  No other
   automatic equivalences should be assumed.

Fixes #15.

Signed-off-by: Felix Fietkau <[email protected]>