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

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-02-01:
+
+- Fixed: scalar arrays (-a) and associative arrays (-A) of a type created by
+  'enum' allowed values not specified by the enum type, corrupting results.
+
+- Fixed: the "${array[@]}" expansion for associative arrays of a type created
+  by 'enum' expanded to random numbers instead of the array's values.
+
 2021-01-30:
 
 - The -x option to the 'command' built-in now causes it to bypass built-ins
@@ -12,7 +20,7 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   a command with many arguments was divided into several command invocations.
 
 - The 2020-08-16 fix is improved with a compile-time feature test that
-  detects if and how the OS uses data alignment in the arguments list,
+  detects if the OS requires extra bytes per argument in the arguments list,
   maximising the efficiency of 'command -x' for the system it runs on.
 
 2021-01-24: