resolvconf: fixes for the compatibility interface

resolvconf: fixes for the compatibility interface

Also use compat_main() when called as `resolvconf`, since the interface
is closer to that of `systemd-resolve`.

Use a heap allocated string to set arg_ifname, since a stack allocated
one would be lost after the function returns. (This last one broke the
case where an interface name was suffixed with a dot, such as in
`resolvconf -a tap0.dhcp`.)

Tested:
  $ build/resolvconf -a nonexistent.abc </etc/resolv.conf
  Unknown interface 'nonexistent': No such device

Fixes #9423.