udp answer from correct address

Send answer from correct address for UDP socket.

Works for IPv4 and IPv6 on Linux, BSD and MacOS.

How to reproduce and test this behavior:
`dig -b ip_address @127.0.0.1 ...`

Solves #277 Multi-NIC is not working properly.


See merge request !492

Avoid duplicate NSEC for Wildcard No Data answer

close #336

See merge request !491

Writing of duplicate keys into keystore

close #365

See merge request !490

The proof for Wildcard No Data consists of two NSEC records: The first
one matches the wildcard record and shows that the QTYPE doesn't exist.
The second one shows there is no better match.

If there are no names below the wildcard in the zone, these two NSEC
records will be the same.

fixes #444 

Use LIBS instead of LDFLAGS to reorder libraries after sources (See https://gist.github.com/darix/2725672008083c9111ff)

Do a numeric syntax check on mapsize values



See merge request !488