s4: LDB DN validity checks

[Matthias Dieter Wallnöfer]

I've realised that at the moment we are doing DN checks at the beginning 
of each LDAP server operation. At the first glance this might be useful, 
but it is only done on exactly this backend. So I tried to remove these 
checks and see what happens: ldap.py and other DN checks passed beside 
one case which I had to patch ("objectclass" commit) also without them. 
That means our LDB modules already perform such checks implicitly - with 
the advantage that this works on each type of external access.

Also the LDB tools enforce such checks at the beginning, but I propose 
to let them in place since these are essential for non-SAM LDB 
databases. So I've fixed them up a little (consistent error messages and 
better return code LDB_ERR_INVALID_DN_SYNTAX).

Branch "master": 
http://gitweb.samba.org/samba.git/?p=mdw/samba.git;a=summary

Cheers,
Matthias