Introduce json_do_string() a function that JSON escapes a string.

Introduce json_do_string() a function that JSON escapes a string.

Implement json_do_printf() using json_do_string() and vasprintf().
json_do_string() only escapes the basic control chars (\b, \f, \n, \r and \t)
other control chars are considered an error. Also the forward slash is
not escaped since the JSON data is not embedded into HTML or XML.
With feedback from tb@ & millert@
OK tb@