resolvectl: Only strip ifname suffixes when being resolvconf
    
    Only treat interface names containing dots specially when resolvectl is
    pretending to be resolvconf to fix
    https://github.com/systemd/systemd/issues/20014 .
    
    Move the special suffix-stripping behaviour of ifname_mangle out to the
    new ifname_resolvconf_mangle to be called from resolvconf only.
    
    (cherry picked from commit 7875170f01991a1d28cfe284cc7075630cd69055)