[patch] ldb in Samba3

Volker,

Here is a patch that adds ldb in Samba3, and adds a groupdp mapping
backend based on ldb. It also splits out the tdb backends into
mapping_tdb.c. That isn't needed, but it makes it a bit clearer what
was done. The backwards compatibility code will be based on that
module (needed before release).

I've only done basic testing on this so far, but it does seem to work.

It assumes you have lib/ldb as a symlink to current ldb from
Samba4. Obviously if committed we will use a svn copy.

Cheers, Tridge