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/tests/enum.sh

@@ -161,5 +161,12 @@ then	enum PARSER2_t=(a b)
 fi
 PATH=/dev/null command -v PARSER2_t >/dev/null && err_exit "PARSER2_t incompletely defined though definition was never executed"
 
+# ======
+# https://github.com/ksh93/ksh/issues/350#issuecomment-982168684
+got=$("$SHELL" -c 'enum trap=(BAD TYPE)' 2>&1)
+exp=': trap: is a special shell builtin'
+[[ $got == *"$exp" ]] || err_exit "enum overrides special builtin" \
+	"(expected match of *$(printf %q "$exp"); got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))

src/cmd/ksh93/tests/types.sh

@@ -565,8 +565,9 @@ unset z
 u_t -a x | read z
 [[ $z == unset ]]  && err_exit 'unset discipline called on type creation'
 
-{ z=$($SHELL 2> /dev/null 'typeset -T foo; typeset -T') ;} 2> /dev/null
-[[ $z == 'typeset -T foo' ]] || err_exit '"typeset -T foo; typeset -T" failed'
+got=$("$SHELL" -c 'typeset -T foo; typeset -T' 2>&1)
+[[ -z $got ]] || err_exit '"typeset -T foo; typeset -T" exposed incomplete type builtin' \
+	"(got $(printf %q "$got"))"
 
 { z=$($SHELL 2> /dev/null 'typeset -T foo=bar; typeset -T') ;} 2> /dev/null
 [[ $z ]] && err_exit '"typeset -T foo=bar" should not creates type foo'
@@ -657,5 +658,12 @@ exp='Subsh_t -a v=((typeset -i x=1) (typeset -i x=2) (typeset -i x=3))'
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 PATH=/dev/null command -v Subsh_t >/dev/null && err_exit "Subsh_t leaked out of subshell"
 
+# ======
+# https://github.com/ksh93/ksh/issues/350#issuecomment-982168684
+got=$("$SHELL" -c 'typeset -T trap=( typeset -i i )' 2>&1)
+exp=': trap: is a special shell builtin'
+[[ $got == *"$exp" ]] || err_exit "typeset -T overrides special builtin" \
+	"(expected match of *$(printf %q "$exp"); got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))