Fix CPU usage reporting

[tklauser]

Two followups for #143:

    Restore CPU usage report format

    Commit 78435f89 changed the existing output format for the CPU usage
    from `cpu usage:` to `cpu usage (all):`.

    This might break existing scripts relying on this format. Revert that
    part of the change and only report the CPU usage for the given period
    with a suffix:

    cpu usage:      0.087%
    cpu usage (1s): 1.42%

and

    Allow to specify CPU usage period as duration string

    The current implementation assumes the given duration in seconds.
    Instead, also allow to specify a duration in the format as expected by
    `time.ParseDuration` and use that if found. If a plain integer is
    specified, it is still parsed as a duration in seconds.