This error was happening on fresh snap install (not upgrade):
```
install: cannot stat '/snap/fwupd/1065/etc/dbus-1/system.d/org.freedesktop.fwupd.conf': No such file or directory
```
Fixes 41a25be6 ("Move D-Bus conf file to datadir/dbus-1/system.d")
The systemd shutdown script gets called after /snap/fwupd/* gets
unmounted meaning it can't be used to do the activation.
Explicitly check that the symlink for /snap/fwupd/current is mounted
when calling the script.
This is intended for devices that it is not safe to immediately activate
the firmware. It may be called at a more convenient time instead.
Both fwupdmgr and fwupdtool support the feature.
- if called at runtime with fwupdmgr it uses the daemon
- during shutdown fwupdtool uses the pending.db to perform this feature.
