r490: - expanded the test suite to test modify and delete operations

r490: - expanded the test suite to test modify and delete operations

- made yet another attempt to make ldb const clean.

- "make test" now runs both the tdb and ldap backend tests, and run the ldbtest utility
  with and without indexing

- added prototypes in ldb.h for ldb_msg_*() public functions
(This used to be commit 01e87406768cb5a98ac8530a2f361a4987a36cd3)