This commit introduces a unified logging strategy, summarized as below:
1. Use `error!` to mark errors or unexpected conditions, e.g., a
`Result::Err` returned from a system call.
2. Use `warn!` to warn about potentially problematic issues, e.g.,
executing a workaround or fake implementation.
3. Use `info!` to show important events (from users' perspective) in
normal execution, e.g., creating/exiting a process/thread.
4. Use `debug!` to track major events in normal execution, e.g., the
high-level arguments of a system call.
5. Use `trace!` to record the most detailed info, e.g., when a system
call enters and exits the LibOS.
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