update strongswan-ikev2 package for ubuntu xenial

[Lars]

Update strongswan-ikev2 package to latest stable release 5.6.3, or backport mentioned security and bug fixes.

[Impact]
 * Several security vulnerabilities have been fixed
   * CVE-2018-6459: Insufficient Input Validation in RSASSA-PSS Signature Parser. For more details see: https://strongswan.org/blog/2018/02/19/strongswan-vulnerability-(cve-2018-6459).html
   * CVE-2018-5388: Insufficient Input Validation in stroke Plugin: For more details see: https://strongswan.org/blog/2018/05/28/strongswan-vulnerability-(cve-2018-5388).html
   * CVE-2018-10811: Missing Initialization of a Variable in IKEv2 Key Derivation. For more details see: https://strongswan.org/blog/2018/05/28/strongswan-vulnerability-(cve-2018-10811).html
 * Packet loss during IKEv2 CHILD_SA rekeying: https://wiki.strongswan.org/issues/1291

[Test Case]

 * Consult links above for detailed information.

[Regression Potential]

 * No regression expected, as all CVEs are security fixes and one bug fix.

[Other Info]

 * n/a

----
# lsb_release -rd
Description: Ubuntu 16.04.4 LTS
Release: 16.04

# apt-cache policy strongswan-ikev2
strongswan-ikev2:
  Installed: 5.3.5-1ubuntu3.5
  Candidate: 5.3.5-1ubuntu3.5

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Christian Ehrhardt  (paelzer)]

Two of the three are already tracked https://people.canonical.com/~ubuntu-security/cve/pkg/strongswan.html it seems.
I'll subscribe the security Team to evaluate and comment here.

[do3meli (d-info-e)]

i am also affected by packet loss during IKEv2 CHILD_SA rekeying [1] and would like to see [2] backported to xenial in addition to the CVE fixes.

[1] https://wiki.strongswan.org/issues/1291
[2] https://github.com/strongswan/strongswan/commit/f8eb636e

[Dani (dan1el1)]

Bump, we would also welcome an update to 5.6.3 for the reasons listed above.

[Simon Déziel (sdeziel)]

Marking as fix released as Ubuntu Focal shipped with strongswan 5.8.2 (> 5.6.3). All supported releases also got the mentioned CVEs fixed.