kbx: Simplify by removing custom memory functions.

* kbx/keybox-util.c (keybox_set_malloc_hooks): Remove.
(_keybox_malloc, _keybox_calloc, keybox_realloc)
(_keybox_free): Remove.
(keybox_file_rename): Remove.  Was not used.
* sm/gpgsm.c (main): Remove call to keybox_set_malloc_hooks.
* kbx/kbxutil.c (main): Ditto.
* kbx/keybox-defs.h: Remove all separate includes.  Include util.h.
remove convenience macros.
* common/logging.h (return_if_fail): New.  Originally from
keybox-defs.h but now using log_debug.
(return_null_if_fail): Ditto.
(return_val_if_fail): Ditto.
(never_reached): Ditto.

Originally the KBX code was written to allow standalone use. However
this required lot of ugliness like separate memory allocators and
such. It also precludes the use of some standard functions from
common due to their use of the common gnupg malloc functions.
Dropping all that makes things easier. Minor disadvantages: the kbx
call done for gpg will now use gcry malloc fucntions and not the
standard malloc functions. This might be a bit slower but removing
them even fixes a possible bug in keybox_tmp_names which is used in
gpg and uses gpg's xfree which is actually gcry_free.

• Signed-off-by: Werner Koch <wk@gnupg.org>