[BSA-116] Security Update for openvpn

Bernhard Schmidt uploaded new packages for openvpn which fixed the
following security problems:

CVE-2017-7479

It was discovered that openvpn did not properly handle the
rollover of packet identifiers. This would allow an authenticated
remote attacker to cause a denial-of-service via application
crash.  

CVE-2017-7508

Guido Vranken discovered that openvpn did not properly handle
specific malformed IPv6 packets. This would allow a remote
attacker to cause a denial-of-service via application crash.

CVE-2017-7520

Guido Vranken discovered that openvpn did not properly handle
clients connecting to an HTTP proxy with NTLMv2
authentication. This would allow a remote attacker to cause a 
denial-of-service via application crash, or potentially leak
sensitive information like the user's proxy password.

CVE-2017-7521

Guido Vranken discovered that openvpn did not properly handle
some x509 extensions. This would allow a remote attacker to cause
a denial-of-service via application crash.

For the jessie-backports distribution the problems have been fixed in
version 2.4.0-6+deb9u1~bpo8+1.