Don't allow 'enum' and 'typeset -T' to override special built-ins

Special builtins are undeleteable for a reason. But 'enum' and
'typeset -T' allow overriding them, causing an inconsistent state.

@JohnoKing writes:
| The behavior is rather buggy, as it appears to successfully
| override normal builtins but fails to delete the special
| builtins, leading to scenarios where both the original builtin
| and type are run:
|
| $ typeset -T eval=(typeset BAD; typeset TYPE)  # This should have failed
| $ eval foo=BAD
| /usr/bin/ksh: eval: line 1: foo: not found
| $ enum trap=(BAD TYPE)   # This also should have failed
| $ trap foo=BAD
| /usr/bin/ksh: trap: condition(s) required
| $ enum umask=(BAD TYPE)
| $ umask foo=BAD
| $ echo $foo
| BAD
|
| # Examples of general bugginess
| $ trap bar=TYPE
| /usr/bin/ksh: trap: condition(s) required
| $ echo $bar
| TYPE
| $ eval var=TYPE
| /usr/bin/ksh: eval: line 1: var: not found
| $ echo $var
| TYPE

This commit fixes the following:

The 'enum' and 'typeset -T' commands are no longer allowed to
override and replace special built-in commands, except for type
definition commands previously created by these commands; these
are already (dis)allowed elsewhere.

A command like 'typeset -T foo_t' without any assignments no longer
creates an incompletely defined 'foo_t' built-in comamnd. Instead,
it is now silently ignored for backwards compatibility. This did
have a regression test checking for it, but I'm changing it because
that's just not a valid use case. An incomplete type definition
command does nothing useful and only crashes the shell when run.

src/cmd/ksh93/bltins/enum.c: b_enum():
- Do not allow overriding non-type special built-ins.

src/cmd/ksh93/sh/name.c: nv_setlist():
- Do not allow 'typeset -T' to override non-type special built-ins.
  To avoid an inconsistent state, this must be checked for while
  processing the assignments list before typeset is really invoked.

src/cmd/ksh93/bltins_typeset.c: b_typeset():
- Only create a type command if sh.envlist is set, i.e., if some
  shell assignment(s) were passed to the 'typeset -T' command.

Progresses: https://github.com/ksh93/ksh/issues/350

src/cmd/ksh93/COMPATIBILITY

@@ -168,6 +168,10 @@ For more details, see the NEWS file and for complete details, see the git log.
 	return value that fits in a signed integer, typically a 32-bit value.
 	Note that $? is truncated to 8 bits when the current (sub)shell exits.
 
+31.	The 'enum' and 'typeset -T' commands are no longer allowed to
+	override and replace special built-in commands, except for type
+	definition commands previously created by these commands.
+
 ____________________________________________________________________________
 
 		KSH-93 VS. KSH-88