privatize ldns library (only used for drill(1))

privatize ldns library (only used for drill(1))

This is the first conversion in a series to rename base libraries
so that they are only used by base assets.  Libraries such as ldns and
ssh have been getting picked up by dports with undesirable results.

These libraries are getting renamed to libprivate_<name>.*.
Coincidentially, FreeBSD-CURRENT is using a similar scheme with
libprivate<name>.*.  FreeBSD Release 10 used the scheme
/usr/lib/private/lib<name>* and DF did not want to follow that.

There may be some breakage for any third party software (including
dports) that have linked to libldns erroneously.  Recompiling and
linking against port libldns should resolve those issues.  Removing
the base libldns.* files is necessary and unavoidable.