systemd-id128: a new tool to print machine/boot/invocation/app-specific ids

systemd-id128: a new tool to print machine/boot/invocation/app-specific ids

The raison d'etre for this program is printing machine-app-specific IDs. We
provide a library function for that, but not a convenient API. We can hardly
ask people to quickly hack their own C programs or call libsystemd through CFFI
in python or another scripting language if they just want to print an ID.

Verb 'new' was already available as 'journalctl --new-id128', but this makes
it more discoverable.

v2:
- rename binary to systemd-id128
- make --app-specific= into a switch that applies to boot-id and machine-id