field_def: pass char ** instead of char * to field_def_array_decode

So that the caller can figure out the end of fields array without
calling mp_next(). Needed for decoding space upgrade format.

While we are at it, let's also
 - Replace MP_ARRAY assertion with diag_set. This makes the function
   easier to use when the MsgPack format isn't guaranteed to be
   MP_ARRAY.
 - Rearrange arguments so that [out] arguments are grouped together.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring