r25942: Make various ldb modules handle an LDB backend that enforces validity

r25942: Make various ldb modules handle an LDB backend that enforces validity

of Base DNs in searches (returning an error of LDB_ERR_NO_SUCH_ENTRY).

We need to handle this if ldb_tdb is to behave correctly compared with
LDAP, as well as if we are using an LDAP backend.

In doing so, I realised that subtree_rename and subtree_delete
(prevention) need rather different wait loops, so it seemed easier to
split it out into it's own module.

I've fixed the licence on both of these modules to be GPLv3.

Andrew Bartlett
(This used to be commit d3894c90f31fb45e038ab478cd9d7d34962d069b)