_dbus_write_uuid_file: factor out function to write a known UUID

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=77941
Reviewed-by: Lennart Poettering
Try to read /etc/machine-id before inventing a new /var/lib/dbus/machine-id

It's least confusing if the two files have the same contents. systemd
already knows how to pick up our /var/lib/dbus/machine-id if it exists
and /etc/machine-id doesn't, but the converse is not currently true.
We should make it true, so that it doesn't matter what order
systemd-machine-id-setup and "dbus-uuidgen --ensure" were
invoked in.

In Debian, systemd currently Recommends dbus, so "dbus-uuidgen --ensure"
will *usually* - but not always! - run first, and the two files will
match. However, if you install systemd without dbus, and then install
dbus later, there will be a mismatch. With this change, it doesn't
matter which one is installed first: whichever one happens to come
first, it will generate the machine ID, and then the other one will
copy it.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=77941
Reviewed-by: Lennart Poettering
Add "Documentation=man:dbus-daemon(1)" line to systemd service

Enhances usability under systemd by making the documentation available
with systemctl status or systemctl help.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=77447
Reviewed-by: Simon McVittie
NEWS for 1.8
Merge branch 'dbus-1.8'

Conflicts:
	NEWS
more NEWS for 1.9
Set argv[0] to dbus-launch to avoid misleading info from /proc

At previous, argv[0] is the full-qualified path of program, however, if
start dbus-launch failed, it will fall back to find one from $PATH,
while keep the argv[0] as the full-qualified path. So we'll get
misleading info from /proc or ps(1) about dbus-launch process.

One simple solution is just hard-code argv[0] to dbus-launch.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=69716
Reviewed-by: Simon McVittie
NEWS