nispor: derive alt-name link match from interface identifier

Route systemd `.link` `[Match]` generation through a single
`gen_systemd_match_rules` that dispatches on the interface identifier,
resolve the identifier in `effective_identifier`, and move the shared
`PermanentMACAddress`/`MACAddress`/`Driver` formatting into
`mac_match_rules`.

When the desired state omits the identifier, `effective_identifier`
falls back to the current interface's identifier. The guard that makes
this safe for stacked interfaces comes next.

Signed-off-by: Josephine Pfeiffer <josie@redhat.com>