Rework `filetype` change, `reload` command and `autosave`

[JoeKar]

• the change of the filetype could cause unexpected behavior depending on global options being reset, while a volatile setting has been used.
• the reload command could cause the same unexpected behavior and additionally doesn't really reload the user settings
• autosave is now aborted in case his value has changed to something smaller or equal to 0.

Fixes #3342