matrix-synapse-py3 debian package doesn't support non-systemd environments

This issue has been migrated from [#9266](https://github.com/matrix-org/synapse/issues/9266).

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**Description:**

I found out [here](https://github.com/matrix-org/synapse/issues/4789#issuecomment-469097511) that Synapse doesn't support other init systems then systemd after unsuccessfully trying to get it to work inside a [Linux-VServer](http://linux-vserver.org/Welcome_to_Linux-VServer.org) container on [Devuan](https://en.wikipedia.org/wiki/Devuan) GNU/Linux.

I think you really should support other init systems like SysVinit because of the following reasons:

- First of all [systemd was discussed very controversial](https://en.wikipedia.org/wiki/Systemd#Reception). ([Debian debate about systemd](https://wiki.debian.org/Debate/initsystem/sysvinit)) It goes against the [original Unix philosophy (the part of software doing only one thing, but doing it well)](https://en.wikipedia.org/wiki/Unix_philosophy#Do_One_Thing_and_Do_It_Well), which GNU/Linux (and [GNU/Hurd](https://en.wikipedia.org/wiki/GNU_Hurd)) have adopted. I also think that complex software is less secure and less stable, because less people are able to check it. As server might have much information about many users, which also more or less rely on the server to work, and is exposed to the internet all the time I think it is crucial for it to be (very) stable and secure (which is also why I use well-tried, simpler (measured on simplicity of understanding the software, not the simplicity of using it) Linux-VServer instead of something like Docker or LXC/LXD by the way).
- Second in order for Matrix to bring people together it should work on as many systems as possible. I for example am not able to simply switch to systemd on my private server, because Linux-VServer doesn't work (well and secure) with systemd and I have a working Cloud, encrypted Pastebin and online whiteboard running on my server in different Linux-VServer containers. I pretty much rely on my self-hosted online services as the reason I am hosting them is to be independent (or at least as independent as possible) from the big concerns like Google or Microsoft.

Considering the elaborated reasons, it would be great if you could support other init systems like SysVinit.

Thanks in advance!

PS: Feel free to move this issue to Dendrite if init system compatibility is easier to implement there or you would like to bring new features directly to the newer version.