python: Add Progspace.objfiles method

This patch adds an objfiles method to the Progspace object, which
returns a sequence of the objfiles associated to that program space.  I
chose a method rather than a property for symmetry with gdb.objfiles().

gdb/ChangeLog:

	* python/py-progspace.c (PSPY_REQUIRE_VALID): New macro.
	(pspy_get_objfiles): New function.
	(progspace_object_methods): New.
	(pspace_object_type): Add tp_methods callback.
	* python/python-internal.h (build_objfiles_list): New
	declaration.
	* python/python.c (build_objfiles_list): New function.
	(gdbpy_objfiles): Implement using build_objfiles_list.
	* NEWS: Mention the Progspace.objfiles method.

gdb/doc/ChangeLog:

	* python.texi (Program Spaces In Python): Document the
	Progspace.objfiles method.
	(Objfiles In Python): Mention that gdb.objfiles() is identical
	to gdb.selected_inferior().progspace.objfiles().

gdb/testsuite/ChangeLog:

	* gdb.python/py-progspace.exp: Test the Progspace.objfiles
	method.

gdb/python/python.c

@@ -1437,10 +1437,10 @@ gdbpy_get_current_objfile (PyObject *unused1, PyObject *unused2)
   return result;
 }
 
-/* Return a sequence holding all the Objfiles.  */
+/* See python-internal.h.  */
 
-static PyObject *
-gdbpy_objfiles (PyObject *unused1, PyObject *unused2)
+gdbpy_ref<>
+build_objfiles_list (program_space *pspace)
 {
   struct objfile *objf;
 
@@ -1448,15 +1448,23 @@ gdbpy_objfiles (PyObject *unused1, PyObject *unused2)
   if (list == NULL)
     return NULL;
 
-  ALL_OBJFILES (objf)
-  {
-    PyObject *item = objfile_to_objfile_object (objf);
+  ALL_PSPACE_OBJFILES (pspace, objf)
+    {
+      PyObject *item = objfile_to_objfile_object (objf);
 
-    if (!item || PyList_Append (list.get (), item) == -1)
-      return NULL;
-  }
+      if (item == nullptr || PyList_Append (list.get (), item) == -1)
+	return NULL;
+    }
 
-  return list.release ();
+  return list;
+}
+
+/* Return a sequence holding all the Objfiles.  */
+
+static PyObject *
+gdbpy_objfiles (PyObject *unused1, PyObject *unused2)
+{
+  return build_objfiles_list (current_program_space).release ();
 }
 
 /* Compute the list of active python type printers and store them in