lib: keepon improving

Improve the name, type and handling of `data->req.keepon`:

- Rename `keepon` to `io_flags`
- make `io_flags` and `uint8_t` and reposition in struct
- Rename `KEEP_*` defines to `REQ_IO_*`, move to request.h
- Replace all direct bit tests to `CURL_REQ_WANT_*` use
- Replace all direct bit manipulations with new macros

Closes #20905

lib/vssh/ssh.h

@@ -162,7 +162,7 @@ struct ssh_conn {
   int secondCreateDirs;         /* counter use by the code to see if the
                                    second attempt has been made to change
                                    to/create a directory */
-  int waitfor;                  /* KEEP_RECV/KEEP_SEND bits overriding
+  int waitfor;                  /* REQ_IO_RECV/REQ_IO_SEND bits overriding
                                    pollset given flags */
   char *slash_pos;              /* used by the SFTP_CREATE_DIRS state */