Revert removal of trigger flag from plperl function hash key.

As noted by Jan Urbanski, this flag is in fact needed to ensure that the
function's input/result conversion functions are set up as expected.

Add a regression test to discourage anyone from making same mistake
in future.

src/pl/plperl/expected/plperl_trigger.out

@@ -266,3 +266,9 @@ SELECT * FROM trigger_test;
  4 | immortal
 (1 row)
 
+CREATE FUNCTION direct_trigger() RETURNS trigger AS $$
+    return;
+$$ LANGUAGE plperl;
+SELECT direct_trigger();
+ERROR:  trigger functions can only be called as triggers
+CONTEXT:  compilation of PL/Perl function "direct_trigger"

src/pl/plperl/plperl.c

@@ -113,7 +113,7 @@ typedef struct plperl_proc_desc
 
 /**********************************************************************
  * For speedy lookup, we maintain a hash table mapping from
- * function OID + user OID to plperl_proc_desc pointers.
+ * function OID + trigger flag + user OID to plperl_proc_desc pointers.
  * The reason the plperl_proc_desc struct isn't directly part of the hash
  * entry is to simplify recovery from errors during compile_plperl_function.
  *
@@ -127,6 +127,11 @@ typedef struct plperl_proc_desc
 typedef struct plperl_proc_key
 {
 	Oid			proc_id;				/* Function OID */
+	/*
+	 * is_trigger is really a bool, but declare as Oid to ensure this struct
+	 * contains no padding
+	 */
+	Oid			is_trigger;				/* is it a trigger function? */
 	Oid			user_id;				/* User calling the function, or 0 */
 } plperl_proc_key;
 
@@ -1955,6 +1960,7 @@ compile_plperl_function(Oid fn_oid, bool is_trigger)
 
 	/* Try to find function in plperl_proc_hash */
 	proc_key.proc_id = fn_oid;
+	proc_key.is_trigger = is_trigger;
 	proc_key.user_id = GetUserId();
 
 	proc_ptr = hash_search(plperl_proc_hash, &proc_key,

src/pl/plperl/sql/plperl_trigger.sql

@@ -128,5 +128,10 @@ FOR EACH ROW EXECUTE PROCEDURE immortal('immortal');
 
 DELETE FROM trigger_test;
 
-
 SELECT * FROM trigger_test;
+
+CREATE FUNCTION direct_trigger() RETURNS trigger AS $$
+    return;
+$$ LANGUAGE plperl;
+
+SELECT direct_trigger();