dist: add scripts to install configuration files (`etc/`, `systemd/`)

[bmr-cymru]

We have a few config files and other bits now that need to go to various places in the file system:

• Our own /etc/snapm/{sanpm.conf,plugins.d/,schedule.d/}
• The systemd timer unit and service files to /usr/lib/systemd/system
• The tmpfiles.d configuration to /usr/lib/systemd/system
• Manual pages to /usr/share/man/man{5,8}

Let's add a script to do this, most likely a Makefile in the project root directory: telling users of pip and git clones to "just run this handy list of six sudo cp commands" is getting old. We can also use this in the RPM spec files and cut back on explicit %{__install} macro use in %install.

This should support idiomatic variables to control the destination path: PREFIX etc.