Refactor.

Refactor.

* gettext-tools/src/its.c (struct its_rule_class_ty): Change parameter names of
methods.
(its_rule_destructor, its_translate_rule_constructor, its_translate_rule_eval,
its_localization_note_rule_constructor, its_localization_note_rule_eval,
its_element_within_text_rule_constructor, its_element_within_text_rule_eval,
its_preserve_space_rule_constructor, its_preserve_space_rule_eval,
its_extension_context_rule_constructor, its_extension_context_rule_eval,
its_extension_escape_rule_constructor, its_extension_escape_rule_eval,
its_rule_destroy): Rename parameter 'pop' to 'rule'.
(its_rule_alloc): Rename local variable 'pop' to 'rule'.