Backport syslinux 6.x for Wheezy

For Tails 1.1, we’ll carry a custom backport, likely with a backported upstream bugfix (#7232 (closed)). But then, once syslinux 6.03-pre5 or later reaches Jessie, we would like to use an official one.

Related issues

Related to #5739 (closed)
Blocked by #7174 (closed)

Original created by @intrigeri on 6563 (Redmine)

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