sysupdate: Split update into acquire and install verbs (#40236)

Using roughly the approach described in
https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/2004#note_2145880.
Basically, copying in-progress downloads to a file/partition with a
predictable prefix, and then moving to a predictable ‘pending’ prefix
when ready to install.

Helps: https://github.com/systemd/systemd/issues/34814