slightly different API.  This was mainly done to get rid of
	the error message "invalid field name: xyzzy" that occured if
	the value of xyzzy was bogus, even if xyzzy itself was a valid
	name.  All callers updated.
(CHK_FLAG_A): Similarly rewritten as chk_flag_a().
(assign): Check for valid field name and valid field content
	separately, so that the "invalid field name: xyzzy" message
	isn't printed just because the content is bad.
(chk_assign): New name for former aux_item_def_check_assign.  New
	arguments: found and good; return status via them.
(chk_flag_a): New function.
(aux_item_def_check_trigger): New arguments: found and good;
	return status via them.
(aux_item_def_check_validate): Ditto.