Fix declaration of $_TD in "strict" trigger functions

This was broken in commit ef19dc6d39 by
the Bunce/Hunsaker/Dunstan team, which moved the declaration from
plperl_create_sub to plperl_call_perl_trigger_func.  This doesn't
actually work because the validator code would not find the variable
declared; and even if you manage to get past the validator, it still
doesn't work because get_sv("_TD", GV_ADD) doesn't have the expected
effect.  The only reason this got beyond testing is that it only fails
in strict mode.

We need to declare it as a global just like %_SHARED; it is simpler than
trying to actually do what the patch initially intended, and is said to
have the same performance benefit.

As a more serious issue, fix $_TD not being properly local()ized,
meaning nested trigger functions would clobber $_TD.

Alex Hunsaker, per test report from Greg Mullane

src/pl/plperl/plperl.c

@@ -1976,8 +1976,11 @@ plperl_call_perl_trigger_func(plperl_proc_desc *desc, FunctionCallInfo fcinfo,
 	ENTER;
 	SAVETMPS;
 
-	TDsv = get_sv("_TD", GV_ADD);
-	SAVESPTR(TDsv);				/* local $_TD */
+	TDsv = get_sv("_TD", 0);
+	if (!TDsv)
+		elog(ERROR, "couldn't fetch $_TD");
+
+	save_item(TDsv);				/* local $_TD */
 	sv_setsv(TDsv, td);
 
 	PUSHMARK(sp);