Add varlink interface for sysinstall (#42261)

Allow graphical installers to use system sysinstall. This will be used
by gnome-setup to install GNOME OS and ideally installers of other
distributions talk to the same varlink interface.

diff --cc src/shared/meson.build
Simple merge

diff --cc src/sysinstall/sysinstall.c
Simple merge

diff --cc units/meson.build
Simple merge