Problem:
    ==============
    By testing `pgrep` with `--ns` option,
    introduced with MDEV-21331, commit fb7c1b94,
    I noted that:
    a) `--ns`  cannot use more than single PID.
    b) `--ns` is returning the processes of the namespace to which supplied PID belongs to.
    So by that sense command `pgrep -x --ns $$ mysqld` will always return an error and skip
    checking of the existing PID of the server.

    Solution:
    ==============
    Suggested solution is to add `--nslist pid`, since `--ns` needs to know in which namespace type it should look for.
    See `pgrep --help` for different namespace types.
    Note also that this works *only* if script is run as a `root` (we have that case here).

    Current PR is a part of:
    1. MDEV-21331: sync preinst and postrm script
    2. MDEV-15718: check for exact mysqld process

    This commit:
    a) fixes fb7c1b94
    b) Closes PR #2068 (obsolete)
    c) Closes PR #2069 (obsolete)

    Thanks Faustin Lammler <faustin@mariadb.org> for testing and verifying
    Reviewed by <>