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
Uploading 100 files of each 1 MB took 20 seconds before. With above fix it takes around 2 seconds now.
10x time improvement in line with pacer's sleep reduction from 100ms to 10ms
