Include the current value of max_stack_depth in stack depth complaints.

I'm mainly interested in finding out what it is on buildfarm machines,
but including the active value in the message seems like good practice
in any case.  Add the info to the HINT, not the ERROR string, so as not
to change the regression tests' expected output.

src/backend/tcop/postgres.c

@@ -3021,8 +3021,9 @@ check_stack_depth(void)
 		ereport(ERROR,
 				(errcode(ERRCODE_STATEMENT_TOO_COMPLEX),
 				 errmsg("stack depth limit exceeded"),
-		 errhint("Increase the configuration parameter \"max_stack_depth\", "
-		   "after ensuring the platform's stack depth limit is adequate.")));
+				 errhint("Increase the configuration parameter \"max_stack_depth\" (currently %dkB), "
+						 "after ensuring the platform's stack depth limit is adequate.",
+						 max_stack_depth)));
 	}
 }