builtin -d should not delete special builtins

The man page for the builtin command says special builtins cannot
be deleted. This wasn't the case though, running `builtin -d` on
a special builtin was deleting it. As an example, the following
set of commands was ending with 'export: not found':

$ builtin -d export
$ export foo=bar

This commit backports the bugfix from ksh93v- (2014-12-24-beta),
which added an error check to prevent special builtins from being
deleted.

src/cmd/ksh93/sh/nvdisc.c:
 - Add an error check to prevent special builtins from being deleted.

src/cmd/ksh93/tests/builtins.sh
 - Add a regression test for using `builtin -d` on special builtins.

NEWS

@@ -6,6 +6,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2020-06-11:
 
+- Fixed a bug that caused running 'builtin -d' on a special builtin to
+  delete it. The man page for the 'builtin' command documents that special
+  builtins cannot be deleted.
+
 - POSIX compliance fix: It is now possible to set shell functions named
   'alias' or 'unalias', overriding the commands by the same names. In
   technical terms, they are now regular builtins, not special builtins.

src/cmd/ksh93/sh/nvdisc.c

@@ -1203,6 +1203,8 @@ Namval_t *sh_addbuiltin(const char *path, Shbltin_f bltin, void *extra)
 		stakseek(offset);
 		if(extra == (void*)1)
 		{
+			if(nv_isattr(np,BLT_SPC))
+				errormsg(SH_DICT,ERROR_exit(1),"cannot delete: %s%s",name,is_spcbuiltin);
 			if(np->nvfun && !nv_isattr(np,NV_NOFREE))
 				free((void*)np->nvfun);
 			dtdelete(sh.bltin_tree,np);

src/cmd/ksh93/tests/builtins.sh

@@ -686,5 +686,11 @@ PATH=$tmp:$PATH $SHELL <<-\EOF || err_exit "'whence' gets wrong path on init"
 	[[ -x $wc ]]
 EOF
 
+# ======
+# `builtin -d` should not delete special builtins
+(builtin -d export 2> /dev/null
+PATH=/dev/null
+whence -q export) || err_exit '`builtin -d` deletes special builtins'
+
 # ======
 exit $((Errors<125?Errors:125))