Improve json_encode error handling

Improve json_encode error handling

json_encode() now returns bool(false) for all possible errors, throws the
respective warning and also sets the respective json_last_error() error
code. Three new error codes have been added:

  * JSON_ERROR_RECURSION
  * JSON_ERROR_INF_OR_NAN
  * JSON_ERROR_UNSUPPORTED_TYPE

To get a partial JSON output instead of bool(false) the option
JSON_PARTIAL_OUTPUT_ON_ERROR can be specified. In this case the invalid
segments will be replaced either by null (for recursion, unsupported type
and invalid JSON) or 0 (for Inf and NaN).

The warning for invalid UTF-8 stays intact and is thrown also with
display_errors = On. If this behavior is undesired this can be remedied
later.