dtprobed: drop libsystemd linkage

[thesamesam]

Using libsystemd purely for sd_notify got a tonne of discussion after the xz backdoor.

At that time, Lennart himself posted saying that projects shouldn't bring in libsystemd purely for the simple sd_notify API:

In the past, I have been telling anyone who wanted to listen that if all you want is sd_notify() then don't bother linking to libsystemd, since the protocol is stable and should be considered the API, not our C wrapper around it.

See https://www.freedesktop.org/software/systemd/man/devel/sd_notify.html#Standalone%20Implementations for an example.

Let's NIH the trivial protocol to drop a hefty library from a privileged, albeit tiny, daemon.

[nickalcock]

Agreed. I've been thinking of this for some time. It's about five lines, and we're doing something similar just to daemonize dtprobed in the absence of systemd anyway.