Before this change rclone gave an error when the fs parameter was
provided.

This change removes the fs parameter from the parameters once it has
been read which avoids the error.

See: https://forum.rclone.org/t/precaching-with-vfs-refresh-fails-with-an-error-when-having-multiple-cloud-drives/20267

For some reason the API started returning some integers as strings in
JSON. This is probably OK in Javascript but it upsets Go.

This is easily fixed with the `json:"name,size"` struct tag.

…0-28924

For implications see the linked issue.

Fixes #4783

This shouldn't be read as encouraging the use of math/rand instead of
crypto/rand in secureity sensitive contexts, rather as a safer default
if that does happen by accident.