rrsig ttl - fix updating from orig_ttl for all output and diff, see below

RFC requires that unlike other rrset types, RRSIG must store different TTL for each rdata, to keep them equal to signed rrset's TTL. But Knot only stores one TTL per rrset, fixing this by using orig_ttl from RRSIG and updating TTL while answer.

This commit also fixes this for knotc zone-read and knotc zone-diff commands.

Moreover, zone update from differences ignores changed TTL-only for RRSIGS. Changed orig_ttl is still diffed as it's part of rdata.