This adds a wrappers for libjansson routines to handle a greater
variety of data types:

        - json_has_key(),
        - json_assert_is_object(),

        - json_add_uint32(),
        - json_add_uint64(),
        - json_add_timespec(),
        - json_add_ntstatus(),
        - json_add_base64(),

        - json_get_int(),
        - json_get_string(),
        - json_get_sid(),
        - json_get_uint64_t(),
        - json_get_uint32_t(),
        - json_get_array_element(),
        - json_array_length(),
        - json_get_timespec(),
        - json_get_ntstatus(),
        - json_get_guid(), and
        - json_get_base64().

Also add helpers ``json_from_{string,FILEp}()`` for decoding JSON
from strings and file handles, respectively. The inputs may be
either JSON objects or arrays. Add a new constructor
``json_new_null()`` to make it more obvious that incoming JSON
values may be of any type.

Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>