Draft: mariabackup should print its process id like the DB server in its error log

It would be nice if some just started mariabackup reports its process id like a DB server does.

Some main use case would be during testing (RQG, MTR might need that too).

Use cases met during RQG testing:

   The source DB server is under high load.

    mariabackup --backup data to some other location is running concurrent.

    The main RQG process detects whatever non expected behavior of

    - mariabackup  (some RQG timeout related to mariabackup gets exceeded)

      Observed example (2024):

      mariabackup --backup could hang endless if some concurrent InnoDB log resizing happens

    - whatever !=  mariabackup

    Than the test including all its running processes (DB server, mariabackup etc.)

    need to be finished and the resources (ports, stuff in filesystem) freed.

    If not the following bad things could happen later

    - The archiving (tar) of the remains of the failing test meets changing files

      --> rotten traces, cores etc.

    - The next test using the same resources (ports etc.) collides with not yet stopped

       DB server, mariabackup etc. and shows mysterious fake errors.

    The main RQG process takes care of its children but the pids of grand children and

    grand grand children are sometimes unknown.

    Process chains look like

    a) main RQG - DB starter (pid is known) - maybe bash  - maybe rr

         - mariadbd (pid in error log + pidfile)

         The current RQG can handle some hanging mariadbd well by attacking

         the mariadbd process and waiting till the DB starter (child) can be reaped.

    b) main RQG - reporter (pid is known)  - maybe bash - maybe rr - mariabackup (pid unknown)

Experiments with

  test setup leading to endless hanging mariabackup --backup

  + mariabackup writing a message about its pid to STDERR

  + new lean code in the RQG component checking mariabackup

were successful.

Experiments with alternative solutions for discovering the pid of mariabackup  via

- fuser <some file opened by mariabackup>

   Observation: rr will open that file too -> more ugly code

   Fear: What to do if mariabackup hangs before that file is opened at all?

- ps or pstree command and

  - walk along the parent - child relationships till reaching mariabackup   or

  - inspect the processes belonging to the processgroup  till reaching ...  or

  - collect all processes running mariabackup (there are concurrent RQG tests)

    and search for specific data in command line content

were made.

But all these alternative solutions are extreme biased to Linux, require rather

ugly Perl code and seem to be quite error prone.

Many thanks in advance