systemd: Expose systemd minor version

systemd patch versions are as important as the major version number;
they indicate security or bug fixes or other behavioural changes between
versions.

Use float64 over float32 as the rounding error with float32 rendered
250.3 as 250.3000030517578 in my testing.

Signed-off-by: Joe Groocock <jgroocock@cloudflare.com>
Signed-off-by: Joe Groocock <me@frebib.net>