networkctl: politely refuse being called from a different netns than the...

networkctl: politely refuse being called from a different netns than the networkd instance we talk to

Otherwise things get very confusing since we mix up netens data from our
client side and from the data we retrieve from networkd.

In the long run we should teach networkctl some switch to operate safely
on other netns, and in that case also determine the right networkd
instance for that namespace.

Fixes: #19236