r7709: - convert ldb to use popt, so that it can interact with the samba

r7709: - convert ldb to use popt, so that it can interact with the samba
  cmdline credentials code (which will be done soon)

- added a ldb_init() call, and changed ldb_connect() to take a ldb
  context. This allows for much better error handling in
  ldb_connect(), and also made the popt conversion easier

- fixed up all the existing backends with the new syntax

- improved error handling in *_connect()

- fixed a crash bug in the new case_fold_required() code

- ensured that ltdb_rename() and all ltdb_search() paths get the read lock

- added a ldb_oom() macro to make it easier to report out of memory
  situations in ldb code