(load_macros): On overwriting a macro, free the old

(load_macros): On overwriting a macro, free the old
definition.
(minput__init): Store Mmap in load_im_info_keys.
(minput_get_title_icon): If the input method has no title, be sure
to return NULL.
(minput_get_description): If the input method has no description,
be sure to return NULL.
(minput_get_variables): Fix documentation.