r19831: Big ldb_dn optimization and interfaces enhancement patch

r19831: Big ldb_dn optimization and interfaces enhancement patch

This patch changes a lot of the code in ldb_dn.c, and also
removes and add a number of manipulation functions around.

The aim is to avoid validating a dn if not necessary as the
validation code is necessarily slow. This is mainly to speed up
internal operations where input is not user generated and so we
can assume the DNs need no validation. The code is designed to
keep the data as a string if possible.

The code is not yet 100% perfect, but pass all the tests so far.
A memleak is certainly present, I'll work on that next.

Simo.
(This used to be commit a580c871d3784602a9cce32d33419e63c8236e63)