Add CVE-2019-6111, CVE-2019-6110 and CVE-2019-6109

Those correspond to the #2, #3 and #4 issues in
https://sintonen.fi/advisories/scp-client-multiple-vulnerabilities.txt
and affecting src:openssh but not fixed as per now upstream.

Only #1 was adressed and is tracked in CVE-2018-20685.