Import c1c0ede32dc00ed619d1cf5fda40a9de43995f3a from ctdb:

   tdb: add TDB_DISALLOW_NESTING and make TDB_ALLOW_NESTING the default behavior

    We need to keep TDB_ALLOW_NESTING as default behavior,
    so that existing code continues to work.

    However we may change the default together with a major version
    number change in future.

    metze
    (cherry picked from samba commit 3b9f19ed919fef2e88b2f92ae541e07bc7379cd1)

    Signed-off-by: Stefan Metzmacher <metze@samba.org>