USN-4934-2: Exim vulnerabilities

USN-4934-1 fixed several vulnerabilities in Exim. This update provides the corresponding update for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM. CVE-2020-28026 only affected Ubuntu 16.04 ESM. Original advisory details: It was discovered that Exim contained multiple security issues. An attacker could use these issues to cause a denial of service, execute arbitrary code remotely, obtain sensitive information, or escalate local privileges.