sysupdate: Prevent a possible invalid partial+pending state on an instance

sysupdate: Prevent a possible invalid partial+pending state on an instance

If the resource code is recursing, it’s possible for one iteration to
set a partial flag, and then a recursive iteration to set a pending flag
(or vice-versa). It doesn’t make sense to have both set at the same time
for a specific instance, so make sure to clear the other flag when
setting one of them.

Add some assertions to make this invariant clearer and easier to debug
if it fails.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>