tools/runqslower.py: Option for --pid or --tid (#2635)

* tools/runqslower.py: Option for --pid or --tid

Closes: #2607

Option for --pid or --tid tracing.
Enable group leader level tracing.
* --pid <pid>:
    will trace every task that has <pid> as group leader.
    Meaning it will trace the parent and its direct children.
* --tid <tid>:
    will trace the given thread
* Reporting TID instead of PID in the report column.
* Update the example file to reflect the TID/PID change