page_cur_set_before_first(), page_cur_set_after_last(),
page_cur_position(): Add const qualifiers to buf_block_t and rec.
A better solution would be to define a const_page_cur_t and a
set of accessors, but it would lead to severe code duplication.

page_rec_get_n_recs_before(): Add const qualifiers.

page_dir_get_nth_slot(): Define as a const-preserving macro.

page_dir_slot_get_rec(), page_dir_slot_get_n_owned(),
page_dir_find_owner_slot(), page_check_dir(): Add const qualifiers.

page_rec_get_next_low(): Add const qualifiers.

page_rec_get_next_const(), page_rec_get_prev_const(): New functions,
based on the const-less page_rec_get_next() and page_rec_get_prev().

page_cur_get_page(), page_cur_get_block(), page_cur_get_page_zip(),
page_cur_get_rec(): Define as const-preserving macros.

page_cur_try_search_shortcut(), page_cur_search_with_match():
Add const qualifiers.

buf_page_get_mutex(): Add a const qualifier to buf_page_t*.

rec_get_next_ptr_const(): Const variant of rec_get_next_ptr().