GH-122890: Fix low-level error handling in `pathlib.Path.copy()` (#122897)

Give unique names to our low-level FD copying functions, and try each one
in turn. Handle errors appropriately for each implementation:

- `fcntl.FICLONE`: suppress `EBADF`, `EOPNOTSUPP`, `ETXTBSY`, `EXDEV`
- `posix._fcopyfile`: suppress `EBADF`, `ENOTSUP`
- `os.copy_file_range`: suppress `ETXTBSY`, `EXDEV`
- `os.sendfile`: suppress `ENOTSOCK`