pid1: add a new SetShowStatus() bus call to override/restore show status mode

The only way to control "ShowStatus" property programmatically was to use the
signal API and wait until the property "ShowStatus" switched to the new value.

This interface is rather cumbersome to use and doesn't allow to temporarily
override the current setting and later restore the overridden value in
race-free manner.

The new method also accepts the empty string as argument which allows to
restore the initial value of ShowStatus, ie the value before it was overridden
by this method.

Fixes: #11447.