Reachable ip

[PritziElias]

I also ran into the issue with multiple interfaces (#25). Starting from #26 I tested it using a setup with multiple interfaces and found some additional scenarios, where I would suggest some further improvements.

• When sending the message, it should be checked, if the address records included in the message can be reached from the destination IP-Address; If the address record can not be reached, the message should not be sent;
  I ran into the problem, that the non-reachable address records were removed from the message, but the message was sent on every available interface; Because of that, services were announced on interfaces where they can not be reached (although the address record was removed);
• When sending the message, records that are not reachable from the subnet of the sender should be removed, if the desired destination IP is null;
  I ran into this problem when calling Announce(). Since there is no defined destination IP when calling announce, every address record was sent over every interface;
• When checking for duplicate messages, the address of the remote endpoint should also be included in the check;
  I ran into this problem when I tried to query the same service from two different interfaces. The query is the same, but as the query comes from a different interface it should be answered nonetheless. The new answer will include new address records, as other addresses may be reachable from the interface of the new querier.
• IsReachable() should return false when the mask is null, because that means no information about the local address could be gathered on the system; the selected local address is probably not present on the system
• IPv6 link-local addresses should be reachable when their scope id matches; therefore a comparison of the Scope-ID has been added

I included these changes and tested everything on my multiple interface setup. Everything seems to work fine for me, but hopefully, someone can crosscheck. Since I am new to the open-source-community I am glad about feedback.
I appreciate your work and would find it very useful if this PR could be merged. It would simplify the work on multiple interface setups a lot.