simplify dns_sdb API

simplify dns_sdb API

SDB is currently (and foreseeably) only used by the named
builtin databases, so it only needs as much of its API as
those databases use.

- removed three flags defined for the SDB API that were always
  set the same by builtin databases.

- there were two different types of lookup functions defined for
  SDB, using slightly different function signatures. since backward
  compatibility is no longer a concern, we can eliminate the 'lookup'
  entry point and rename 'lookup2' to 'lookup'.

- removed the 'allnodes' entry point and all database iterator
  implementation code

- removed dns_sdb_putnamedrr() and dns_sdb_putnamedrdata() since
  they were never used.