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.
build-sys: "link" libsystemd-id128 against libsd-daemon-internal

georgem> libsystemd-id128.so: undefined reference to `sd_listen_fds'

In some toolchains (--as-needed not used or not working), the
toolchain doesn't drop this dependency. It is introduced because
sd-id128.so is linked against sd-shared.la, and some functions therein
use libsystemd-daemon, but libsd-id128 doesn't use any of those
functions.

This results in no change in libsystemd-id128.so when the unused
symbols are properly stripped.