Correctly block invalid values for arrays of an enum type

This fixes part of https://github.com/ksh93/ksh/issues/87:

Scalar arrays (-a) and associative arrays (-A) of a type created by
'enum' did not consistently block values not specified by the enum
type, yielding corrupted results.

An expansion of type "${array[@]}" yielded random numbers instead
of values for associative arrays of a type created by 'enum'.

This does not yet fix another problem: ${array[@]} does not yield
all values for associative enum arrays.

src/cmd/ksh93/bltins/enum.c: put_enum():
- Always throw an error if the value is not in the list of possible
  values for an enum type. Remove incorrect check for the NV_NOFREE
  flag. Whatever that was meant to accomplish, I've no idea.

src/cmd/ksh93/sh/array.c: nv_arraysettype():
- Instead of sh_eval()ing a shell assignment, use nv_putval()
  directly. Also use the stack (see src/lib/libast/man/stk.3)
  instead of malloc to save the value; it's faster and will be
  auto-freed at some point. This shortens the function and makes it
  faster by not entering into a whole new shell context -- which
  also fixes another problem: the error message from put_enum()
  didn't cause the shell to exit for indexed enum arrays.

src/cmd/ksh93/sh/name.c: nv_setlist():
- Apply a patch from David Korn that correctly sets the data type
  for associative arrays, fixing the ${array[@]} expansion yielding
  random numbers. Thanks to @JohnoKing for the pointer.
  https://github.com/ksh93/ksh/issues/87#issuecomment-662613887
  https://www.mail-archive.com/ast-developers@lists.research.att.com/msg00697.html

src/cmd/ksh93/tests/enum.sh:
- Add tests checking that invalid values are correctly blocked for
  indexed and associative arrays of an enum type.

Makes progress on: https://github.com/ksh93/ksh/issues/87

src/cmd/ksh93/bltins/enum.c

@@ -150,8 +150,7 @@ static void put_enum(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 		}
 		i++;
 	}
-	if(nv_isattr(np,NV_NOFREE))
-		error(ERROR_exit(1), "%s: invalid value %s",nv_name(np),val);
+	error(ERROR_exit(1), "%s: invalid value %s",nv_name(np),val);
 }
 
 static char* get_enum(register Namval_t* np, Namfun_t *fp)