This introduces a new fs.Option flag, Sensitive and uses this along
with IsPassword to redact the info in the config file for support
purposes.
It adds this flag into backends where appropriate. It was necessary to
add oauthutil.SharedOptions to some backends as they were missing
them.
Fixes#5209
Before this change, the Pikpak backend would always download
the first media item whenever possible, regardless of whether
or not it was the original contents.
Now we check the validity of a media link using the `fid`
parameter in the link URL.
Fixes#6992
Before this change the pikpak backend changed the global
--multi-thread-streams flag which wasn't desirable.
Now the machinery is in place to use the NoMultiThreading feature flag
instead.
Fixes#6915
