Remove --memstats feature

Remove --memstats feature

The ``--mememstat`` was largely undocumented and there is no known
user of this feature.  This feature provided very limited statistics
(number of users, link bytes read/written) and we do not except any
usage because of this.

The only documentation was a mention in --help without any mention of
the (binary) format of the mmap file or other usage instructions.

This deals also with issues reported by zeropath regarding potentially
insecure handling of the file permission of the memory mapped file.

Reported-by: Joshua Rogers <contact@joshua.hu>
Found-by: ZeroPath (https://zeropath.com/)
Change-Id: I5f57e7bf52e3f6289462ef05e1f6e81ab0133d0d
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1329
Message-Id: <20251029163849.446-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34021.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>