Harmonize some more function parameter names.

Make sure that function declarations use names that exactly match the
corresponding names from function definitions in a few places.  These
inconsistencies were all introduced relatively recently, after the code
base had parameter name mismatches fixed in bulk (see commits starting
with commits 4274dc22 and 035ce1fe).

pg_bsd_indent still has a couple of similar inconsistencies, which I
(pgeoghegan) have left untouched for now.

Like all earlier commits that cleaned up function parameter names, this
commit was written with help from clang-tidy.

src/pl/plpython/plpy_resultobject.c

@@ -20,7 +20,7 @@ static Py_ssize_t PLy_result_length(PyObject *arg);
 static PyObject *PLy_result_item(PyObject *arg, Py_ssize_t idx);
 static PyObject *PLy_result_str(PyObject *arg);
 static PyObject *PLy_result_subscript(PyObject *arg, PyObject *item);
-static int	PLy_result_ass_subscript(PyObject *self, PyObject *item, PyObject *value);
+static int	PLy_result_ass_subscript(PyObject *arg, PyObject *item, PyObject *value);
 
 static char PLy_result_doc[] = "Results of a PostgreSQL query";