resolved: turn off that a search domain is derived from the host's fqdn

resolved: turn off that a search domain is derived from the host's fqdn

If the hostname of a system is set to an fqdn, glibc traditionally
derives a search domain from it if none is explicitly configured.

This is a bit weird, and we currently don't do that in our own search
path logic.

Following #17193 let's turn this behaviour off for now.

Yes, this has a slight chance of pissing people off who think this
behaviour is good. If this is indeed an issue, we can revisit the issue
but in that case if we readd the concept we should do it properly:
derive the search domain from the fqdn in our codebase too and report it
in resolvectl, and in our generated stub files. But I have the suspicion
most people who set the hostname to an fqdn aren#t even aware of this
behaviour nor want it, so let's wait until people complain.

Fixes: #17193