Add missing libdns++ rdata files for the distribution (this
fixes distcheck error). Change three generated libdns++
headers to "nodist" so they aren't included in the distribution
(they were mistakenly included in last tarball).

This fixes two immediate problems:

If you do a distcheck build or create a tarball it would mistakenly
include the rdataclass.h, rrclass.h, and rrtype.h headers.
They are supposed to be generated at build time.

Also the rp_17 rdata files were not included in the distribution,
so building would fail because rdataclass didn't define the RP class.

This update also includes the nsec_bitmap rdata files. (I didn't
see any problem related to this, but I assume they were meant to
be shipped.)

I added two generated files to BUILT_SOURCES. Not necessarily
required, but this implies that the target for these should be
done before the regular build (so this makes sure the generated
files are created).

Discussed a little on jabber on Friday.

This should fix the existing distcheck autobuilder failures.