network: do not assume the highest priority when Priority= is unspecified

network: do not assume the highest priority when Priority= is unspecified

Previously, when Priority= is unspecified, networkd configured the rule with
the highest (=0) priority. This commit makes networkd distinguish the case
the setting is unspecified and one explicitly specified as Priority=0.

Note.
1) If the priority is unspecified on configure, then kernel dynamically picks
   a priority for the rule.
2) The new behavior is consistent with 'ip rule' command.

Replaces #15606.