Avoid side effects by using own logger instance
- Importing fs/log only sets rclone's private logger via fs.SetLogger,
so internal rclone logging works from the moment the package is
imported but the process-wide slog default is left untouched.
- slog.SetDefault and slog.SetLogLoggerLevel move into InitLogging,
which is called explicitly from the CLI (cmd/cmd.go), the librclone
wrapper and the integration test framework. So rclone-as-a-program
keeps capturing log.Print/log.Fatal and slog.Default() output as
before.
Library consumers that import fs/log without calling InitLogging now
keep their own slog default and can safely route rclone output back
into it via log.Handler.SetOutput without recursing.
Fixes#8907
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>
