r17821: changed ldb_search() and the ldbsearch command line utility to

r17821: changed ldb_search() and the ldbsearch command line utility to
automatically work out the basedn when basedn==NULL. The basedn is
fetched from the rootDSE defaultNamingContext value (if there is one)

This means we don't have to have the defaultNamingContext logic in
lots of places. It makes a lot of sense to me to have basedn==NULL
mean "use the default, as given by the database"

Note that explicitly specifing a basedn of '' is not the same thing,
and will not trigger this code

The baseDN is cached in a ldb opaque, so we only have to fetch it once
(This used to be commit 5d1b66b68fc517ce684f75e466ed5f25e46857d5)