The filtering and display of pids in the runqlat tool was not correct.
Internally, the kernel keeps pid and tgid, which correspond to thread-id
and user process-id, respectively. The runqlat tool was filtering and
displaying pid instead of tgid.

Change -P and -p options to filter by tgid, and add a new option to give
a breakdown by pid (thread-id).

Update the docs with the -L option.