resolvconf-compat: make "-u" operation a NOP

resolvconf-compat: make "-u" operation a NOP

According to the various man pages of "resolvconf" the -u switch is for:

"-u Just run the update scripts (if updating is enabled)."

"-u Force resolvconf to update all its subscribers. resolvconf does not
    update the subscribers when adding a resolv.conf that matches what
    it already has for that interface."

We have no "subscribers", we ourselves are the only "subscriber" we
support. Hence it's probably better to ignore such a request and make it
a NOP, then to fail.

Fixes: #20748