Before this change it was possible to make a remote with an invalid
name in the config file, either manually or with `rclone config
create` (but not with `rclone config`).
When this remote was used, because it was invalid, rclone would
presume this remote name was a local directory for a very suprising
user experience!
This change checks remote names more carefully and returns errors
- when the user tries to use an invalid remote name on the command line
- when an invalid remote name is used in `rclone config create/update/password`
- when the user tries to enter an invalid remote name in `rclone config`
This does not prevent the user entering a remote name with invalid
characters in the config manually, but such a remote will fail
immediately when it is used on the command line.
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