resolvconf-compat: first parse provided interface name as is

resolvconf-compat: first parse provided interface name as is

Then, try to drop multiple protocol specifiers at the end.

Strictly speaking, this breaks backward compatibility:
if eth0 and eth0.42 exists, then previously,
    echo 'nameserver 192.168.0.1' | resolvconf -a eth0.42
adds the DNS server to eth0 instead of eth0.42, as we unconditionally
dropped the specifier after the last dot, and
    echo 'nameserver 192.168.0.1' | resolvconf -a eth0.42.dhcp
adds the DNS server to eth0.42. However, with this commit, now
the both commands add the DNS server to eth0.42. But, hopefully,
this should be preferable behavior.

Fixes #25032.