systemd: read units from /etc/systemd-mutable

This allows for persistent units managed outside of configuration.nix

(cherry picked from commit e3825b47ec76b2178321bb9da362f7308076b902)
systemd: Apply upstream fixes

In particular, upstream commit 3072eecf3c714e3ed88c882623c40aca15445f64
fixes 15-character network interface names (as generated by ID_NET_NAME_MAC).

(cherry picked from commit e47ed98f9b1ebe78d70ba650c09865a9670fd372)