sysupdate: Add partial/pending flags to UpdateSet

sysupdate: Add partial/pending flags to UpdateSet

This commit adds the flags and some basic formatting/printing of them.
Following commits will integrate them into the update/acquire/install
logic.

`UPDATE_PARTIAL` is set if any of the instances in the `UpdateSet` are
partial, i.e. have been partially downloaded.

`UPDATE_PENDING` is set if any of the instances in the `UpdateSet` are
pending, i.e. have been acquired (downloaded) but not yet installed.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://github.com/systemd/systemd/issues/34814