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.
NEWS: update NEWS file according to most recent changes in git
NEWS: mention that we need a new user systemd-timesync
build-sys: use glibc's xattr support instead of requiring libattr
virt: rework container detection logic

Instead of accessing /proc/1/environ directly, trying to read the
$container variable from it, let's make PID 1 save the contents of that
variable to /run/systemd/container. This allows us to detect containers
without the need for CAP_SYS_PTRACE, which allows us to drop it from a
number of daemons and from the file capabilities of systemd-detect-virt.

Also, don't consider chroot a container technology anymore. After all,
we don't consider file system namespaces container technology anymore,
and hence chroot() should be considered a container even less.
Merge branch 'master' of https://github.com/systemd/systemd