wait-online: make manager_link_is_online() return 0 when in unmanaged state

wait-online: make manager_link_is_online() return 0 when in unmanaged state

Previously, even if a link is in unmanaged state, the function may
returns positive value. So, even if all managed links are in the configured
sate but do not satisfy the online criteria, e.g., IPv4 address state,
then wait-online finishes with positive value.

This makes the function always return 0 for unmanaged state. So, at
least one managed link must satisfies the online criteria.

This also adds more comments and debugging logs.

Fixes #22246.