Adding systemd to postmarketOS

The impression I've gotten so far is that musl don't want some things (like the printf extensions) for various reasons, POSIX being a motivating factor. Worth noting that systemd isn't explicitly tied to glibc, and for some things like basename() they're unhappy with any existing approach and use their own instead.

We expect that we'll have to carry some stuff in an external library if it's clear that neither project is willing to take it. Lennart has blessed this approach at least (we can have this dependency expressed in systemd). As things go it could certainly be worse, hopefully we can start finding a constructive way forward as frankly this whole situation is just detrimental to the ecosystem as a whole.