This implements --auth-proxy for serve s3. In addition it:
* add listbuckets tests with and without authProxy
* use auth proxy test framework
* servetest: implement workaround for #7454
* update github.com/rclone/gofakes3 to fix race condition
This also
- move in use options (Opt) from vfsflags to vfscommon
- change os.FileMode to vfscommon.FileMode in parameters
- rework vfscommon.FileMode and add tests
- Changes
- Rename `--s3-authkey` to `--auth-key` to get it out of the s3 backend namespace
- Enable `Content-MD5` integrity checks
- Remove locking after code audit
- Documentation
- Factor out documentation into seperate file
- Add Quickstart to docs
- Add Bugs section to docs
- Add experimental tag to docs
- Add rclone provider to s3 backend docs
- Fixes
- Correct quirks in s3 backend
- Change fmt.Printlns into fs.Logs
- Make metadata storage per backend not global
- Log on startup if anonymous access is enabled
- Coding style fixes
- rename fs to vfs to save confusion with the rest of rclone code
- rename db to b for *s3Backend
Fixes#7062
- add context to log and fallthrough to error log level
- test: use rclone random lib to generate random strings
- calculate hash from vfs cache if file is uploading
- add server started log with server url
- remove md5 hasher
