nm:parse-nm: handle differing ip6-privacy default value

[slyon]

Description

Commit 64e381e (#244) changed netplan's
behaviour to always write out the ipv6-privacy value (default to 0=off),
in order to comply with what is written in our documentation and how it is
implemented in the networkd backend.

This is a problem for our NetworkManager integration, as NM's default value
is -1=unknown. Netplan only supports values 0=off and 2=on while NM supports
-1, 0, 1 and 2. We need to handle unsupported cases (-1 and 1) via passthrough.

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[codecov-commenter]

Codecov Report

Merging #263 (c7e6f31) into main (7030ca9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #263   +/-   ##
=======================================
  Coverage   99.07%   99.07%           
=======================================
  Files          61       61           
  Lines       10837    10860   +23     
=======================================
+ Hits        10737    10760   +23     
  Misses        100      100           

Impacted Files	Coverage Δ
src/nm.c	100.00% <100.00%> (ø)
src/parse-nm.c	100.00% <100.00%> (ø)
tests/generator/test_passthrough.py	100.00% <100.00%> (ø)
tests/parser/test_keyfile.py	100.00% <100.00%> (ø)

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[schopin-pro]

LGTM