Simplify cJSON and add new macros.

* src/cJSON.h: Re-indented.
(cjson_t): New.
(cjson_is_): New macros.
* src/cJSON.c: Re-indented.  Include errno.h and our util.h.
(ep): Remove global var.
(cJSON_GetErrorPtr): Remove.
(cJSON_strcasecmp): Cast args for use with tolower.
(cJSON_malloc, cJSON_free): Remove.  Change callers to use xtrymalloc
and xfree.
(cJSON_InitHooks): Remove.
(cJSON_strdup): Remove.  Change callers to use xtrystrdup.
(cJSON_New_Item): Simplify.
(cJSON_ParseWithOpts, cJSON_Parse): Add arg R_ERROFF.
(parse_string, parse_value, parse_array, parse_object): Add arg EP.

cJSON has been taken from cjson-code-58.zip. The README file has been
renamed to cJSON.readme and the files have been changed to GNU coding
standards. Because that parser is small enough to be source copied it
does not make sense to treat it as a library and I changed the memory
allocation functions to the usual xmalloc ones. The only external
dependency now is out util.h which declares those functions.

The lowercase cjson_t better fits into our coding style as well as the
new macros. Thanks to Dave Gamble for this nice parser.