remove libsystemd dependency

[cnt0]

(I messed stuff up with git branches so decided to create a new PR instead of the old one)

Hello. This PR introduces home-baked sd_booted and sd_notify functions, so it's now possible to completely remove the libsystemd dependency as per Lennart Poettering's advice

I'd recommend apps to just embed their own minimal version of the sd_notify() protocol, in their native language. It's trivial, and nowadays even in crazy languages like Java you have AF_UNIX support that is sufficient to implement it in a few lines of code.

I copied sd_booted almost as-is from libsystemd. As for sd_notify, I've been looking at python implementation and its documented behavior.

This code is entirely untested, other than "it compiles", so please test it before merging (if you'd like to merge this PR, that is)

[cnt0]

#5582 (comment) - also adressed this review

[vaxerski]

lgtm

[vaxerski]

but yeah needs testing + nix fails @fufexan