Added function for comparing RRSet TTLs.

src/knot/dnssec/zone-sign.c

src/knot/updates/ddns.c

src/knot/updates/xfr-in.c

src/knot/zone/node.c

src/knot/zone/semantic-check.c

src/libknot/rrset.c

src/libknot/rrset.h