FR pgcollect: add `‐-max-size` to stop profiling early

[GitMensch]

‐limit-size: Stop profiling when the collected data file size (in MBs) crosses the limit n.

note: if/when #4 is done and enabled it should "of course" limit to the compressed size (if that is known to the application)

[ostash]

This needs to be designed properly:

• when profiling by PID we can detach from the process after reaching the limit;
• when profiling by forking it is not clear what to do: kill? leave?

[GitMensch]

I've just rechecked what perf record does with

       --max-size=<size>
           Limit the sample data max size, <size> is expected to be a
           number with appended unit character - B/K/M/G

when attached: detaches, when cmd is specified: sends it a SIGTERM.

Of course there could be an identifier or extra option that says "detach instead of terminate".