Problem:
followup to 55ceb314ca#39478
`:oldfiles` and swapfile `:recover` do not delegate to `vim.ui.select`.
Solution:
- Delegate to `vim.ui.select`.
- Fix a long-standing `recover_names` bug where `concat_fnames(dir_name,
files[i], true)` produced malformed `<dir>//<dir>/<file>` paths (also
fixes `swapfilelist()`).