ldb_tdb: Make sure max_key_length != 0 requires a GUID index mode

ldb_tdb: Make sure max_key_length != 0 requires a GUID index mode

We need to enforce the GUID index mode so end-users do not get a supprise
in mid-operation and we enforce a max key length of 511 so that the
index key trunctation is done correctly.

Otherwise the DB will appear to work until a very long key (DN or index)
is used, after which it will be sad.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>