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 7875170f)