ldb: version 2.4 will be used for Samba 4.15

ldb: version 2.4 will be used for Samba 4.15

- Improve calculate_popt_array_length()
- Use C99 initializers for builtin_popt_options[]
- pyldb: Fix Message.items() for a message containing elements
- pyldb: Add test for Message.items()
- tests: Use ldbsearch '--scope instead of '-s'
- pyldb: fix a typo
- Change page size of guidindexpackv1.ldb
- Use a 1MiB lmdb so the test also passes on aarch64 CentOS stream
- attrib_handler casefold: simplify space dropping
- fix ldb_comparison_fold off-by-one overrun
- CVE-2020-27840: pytests: move Dn.validate test to ldb
- CVE-2020-27840 ldb_dn: avoid head corruption in ldb_dn_explode
- CVE-2021-20277 ldb/attrib_handlers casefold: stay in bounds
- CVE-2021-20277 ldb tests: ldb_match tests with extra spaces
- improve comments for ldb_module_connect_backend()
- test/ldb_tdb: correct introductory comments
- ldb.h: remove undefined async_ctx function signatures
- correct comments in attrib_handers val_to_int64
- dn tests use cmocka print functions
- ldb_match: remove redundant check
- add tests for ldb_wildcard_compare
- ldb_match: trailing chunk must match end of string
- pyldb: catch potential overflow error in py_timestring
- ldb: remove some 'if PY3's in tests
- Add missing break in switch statement

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