hostnamed: Fix the way that static and transient host names interact

hostnamed: Fix the way that static and transient host names interact

It is almost always incorrect to allow DHCP or other sources of
transient host names to override an explicitly configured static host
name.

This commit changes things so that if a static host name is set, this
will override the transient host name (eg: provided via DHCP). Transient
host names can still be used to provide host names for machines that have
not been explicitly configured with a static host name.

The exception to this rule is if the static host name is set to
"localhost". In those cases we act as if no
static host name has been explicitly set.

As discussed elsewhere, systemd may want to have an fd based ownership
of the transient name. That part is not included in this commit.