Shim has had a hard time with loading updates from arguments in the
most recent release and this isn't the first time that happened.
Give distros and users an escape hatch that will allow using GRUB
instead.
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.