dns/nsd: Update to 4.1.20

This release fixes memory leaks when reading zonefiles
and processing zone transfers.

4.1.20

BUG FIXES:

• Fix memory leak in zone file read of unknown rr formatted RRs.
• Fix memory leak when rehashing nsec3 after axfr or zonefile read, in the selectively allocated precompiled nsec3 hashes.

Also changed to DISTVERSION

Submitted by: jaap@NLnetLabs.nl (maintainer)
Approved by: tcberner (mentor, implicit)