Check if SystemPrompter D-Bus interface is available

Balló Györgyrequested to merge
bgyorgy/pinentry:dbus-check into main

pinentry-gnome3 requires an additional check to ensure that the D-Bus interface org.gnome.keyring.SystemPrompter is available on the system. The interface is provided within GNOME Shell internally, while other desktop environments could use gcr-prompter provided by the gcr package.

Closes: #7 (closed)

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