packaging: cope with unset ID and VERSION_ID

packaging: cope with unset ID and VERSION_ID

There is no guarantee that ID and VERSION_ID are set in /etc/os-release.
Provide defaults for ID and VERSION_ID so that spread runs with _some_ system
name. Spread rejects system names with two consecutive dashes, so use a
special-case for unset VERSION_ID since we know it is used in a context of the
Debian archive.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
Gbp-Pq: Name 0016-PATCH-15-18-packaging-cope-with-unset-ID-and-VERSION.patch