fs: make sure backends with additional config have a different name #4996

Backends for which additional config is detected (in the config string
or on the command line or as environment variables) will gain a suffix
`{XXXXX}` where `XXXX` is a base64 encoded md5hash of the config
string.

This fixes backend caching with config string remotes.

This much requested feature now works properly:

    rclone copy -vv drive,shared_with_me:file.txt drive:

docs/content/docs.md

@@ -285,6 +285,31 @@ does not work on Windows.)
 
     rclone copy ':http,url="https://example.com":path/to/dir' /tmp/dir
 
+#### Connection strings, config and logging
+
+If you supply extra configuration to a backend by command line flag,
+environment variable or connection string then rclone will add a
+suffix based on the hash of the config to the name of the remote, eg
+
+    rclone -vv lsf --s3-chunk-size 20M s3:
+
+Has the log message
+
+    DEBUG : s3: detected overridden config - adding "{Srj1p}" suffix to name
+
+This is so rclone can tell the modified remote apart from the
+unmodified remote when caching the backends.
+
+This should only be noticeable in the logs.
+
+This means that on the fly backends such as
+
+    rclone -vv lsf :s3,env_auth:
+
+Will get their own names
+
+    DEBUG : :s3: detected overridden config - adding "{YTu53}" suffix to name
+
 ### Valid remote names
 
  - Remote names may only contain 0-9, A-Z ,a-z ,_ , - and space.