Expand the pack/unpack API, implement unpack flags

Expand the pack/unpack API, implement unpack flags

Expand the pack/unpack API: json_(un)pack is the simple version with
no error output, json_(un)pack_ex has error output and flags,
json_v(un)pack_ex is a va_list version of json_(un)pack_ex.

Implement unpacking flags:

- JSON_UNPACK_ONLY turns off extra validation, i.e. array and object
  unpacking doesn't check that all items are unpacked. This is really
  just convenience for not adding '*' after each object and array.

- JSON_VALIDATE_ONLY turns off unpacking, i.e. no varargs are expected
  and nothing is unpacked.