Multi-realm KDC null deref [CVE-2013-1418]

If a KDC serves multiple realms, certain requests can cause
setup_server_realm() to dereference a null pointer, crashing the KDC.

CVSSv2: AV:N/AC:M/Au:N/C:N/I:N/A:P/E:POC/RL:OF/RC:C

A related but more minor vulnerability requires authentication to
exploit, and is only present if a third-party KDC database module can
dereference a null pointer under certain conditions.

(cherry picked from commit 5d2d9a1abe46a2c1a8614d4672d08d9d30a5f8bf)

ticket: 7755
version_fixed: 1.12
status: resolved
Clarify realm and dbmodules configuration docs

In kdc_conf.rst, add examples showing how to configure a realm
parameter and a database parameter.  Document that the default DB
configuration section is the realm name, and use that in the example.
Move the db_module_dir description to the end of the [dbmodules]
documentation since it is rarely used and could confuse a reader about
the usual structure of the section.

(cherry picked from commit 689d769c10c53bd4fa40e82421c89b96cc86cbae)

ticket: 7759
version_fixed: 1.12
status: resolved