network: introduce link_should_mark_config()

network: introduce link_should_mark_config()

This split-out common logic from link_drop_routes() and friends.

This is mostly a refactoring, and not change behavior in most cases.
But slightly change behavior for how foreign nexthops and routing
policy rules are managed.

E.g. when KeepConfiguration=static, previously all foreign nexthops and
routing policy rules were kept, but now only foreign nexthops and rules
with RTPROT_STATIC are kept and others are dropped.

Similary, when KeepConfiguration=dynamic, previously all foreign nexthops
and rules were removed, but now foreign configs with a dynamic protocol
e.g. RTPROT_DHCP are kept, and still configs with RTPROT_STATIC are
dropped.

Currently, we do not set/get/manage protocol for neighbor entries.
Hence, the logic of managing foreign neighbor entries is unchanged.