resolvctl: only remove protocol after last dot when mangling ifname for resolvconf

resolvctl: only remove protocol after last dot when mangling ifname for resolvconf

ifname_resolvconf_mangle is supposed to remove protocol suffixes like .dhcp
from interface names. But this removed also valid parts of the ifname like
VLAN IDs, e.g. enp2s0u4.72.dhcp -> enp2s0u4 instead of enp2s0u4.72

After this change, everything behind the last dot is removed instead of the first.