ostree-remount: Remount /etc rw if needed

ostree-remount: Remount /etc rw if needed

When we remount `/sysroot` as read-only, we also make `/etc` read-only.
This is usually OK because we then remount `/var` read-write, which also
flips `/etc` back to read-write... unless `/var` is a separate
filesystem and not a bind-mount to the stateroot `/var`.

Fix this by just remounting `/etc` read-write in the read-only sysroot
case.

Eventually, I think we should rework this to set everything up the way
we want from the initramfs (#2115). This would also eliminate the window
during which `/etc` is read-only while `ostree-remount` runs.