machine-id: fall back to reading /etc/machine-id if D-Bus machine ID is not available

machine-id: fall back to reading /etc/machine-id if D-Bus machine ID is not available

Newer systemd-based systems support a global machine ID with the same
metrics as the D-Bus machine ID, but more powerful semantics (for
example on ro media) in /etc/machine-id.

If the D-Bus machine ID cannot be read, fall back to the systemd machine
ID.

This is a first step towards allowing D-Bus to be started up during
early boot where /var is not available.

[plus a whitespace fix -smcv]

Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Colin Walters <walters@verbum.org>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=35228