hostnamectl: if somebody invokes 'hostnamectl set-hostname' with a valid internet hostname unset the pretty name

If people are unaware or uninterested in the concept of pretty host
names, and simply invoke "hostnamectl set-hostname" for a valid internet
host name, then use this as indication to unset the pretty host name and
only set the static/dynamic one.

This also allows fqdn, hence "hostnamectl set-hostname www.foobar.com"
will just work if people really insist on using fqdns as hostnames.