Add a limit to the number of RRs in RRSets

Add a limit to the number of RRs in RRSets

Previously, the number of RRs in the RRSets were internally unlimited.
As the data structure that holds the RRs is just a linked list, and
there are places where we just walk through all of the RRs, adding an
RRSet with huge number of RRs inside would slow down processing of said
RRSets.

The fix for end-of-life branches make the limit compile-time only for
simplicity and the limit can be changed at the compile time by adding
following define to CFLAGS:

    -DDNS_RDATASET_MAX_RECORDS=<limit>

(cherry picked from commit c5c4d00c38530390c9e1ae4c98b65fbbadfe9e5e)
(cherry picked from commit 7f705778af729ada7fec36ac4b456c73329bd996)