Spawn each tab into a new cgroup for systemd

[imciner2]

Expected behaviour

The terminal launches each tab into its own systemd scope (viewable using systemd-cgls) if compiled with systemd support. This way when system-oomd is used, each tab can be killed separately and it won't kill the entire terminal when the system reaches a high memory pressure.

A similar feature was requested in gnome-terminal a few years ago (while they were switching to using cgroups) here: https://gitlab.gnome.org/GNOME/gnome-terminal/-/issues/206.

Actual behaviour

All the terminal tabs and processes are lumped into the same scope as the main terminal process.

Steps to reproduce the behaviour

Run systemd-cgls and see that mate-terminal and bash and any other programs launched in a terminal are all in the same scope.

MATE general version

1.26.0

Package version

mate-terminal-1.26.0-1.fc35.x86_64

Linux Distribution

Fedora

Link to bugreport of your Distribution (requirement)

(none directly, but this is related to the Fedora issues with system-oomd and mate-desktop being killed, such as https://bugzilla.redhat.com/show_bug.cgi?id=2068699 and https://bugzilla.redhat.com/show_bug.cgi?id=2078108#c1).