pinentry: try to use all gtk/qt backends, use ldd to check deps

Currently, pinentry is hardcoded to use gtk2 backend, and checks for hardcoded library. This doesn't seem graceful, and gtk2 being obsolete for years makes the situation worse, as most of the cases people would just get the curses pinentry as fallback.

Let's refrain from that practice, and use ldd to decide which backends might be usable.

Note that the qt6 backend takes precedence over gnome3 here, because the latter requires gcr which is less commonly installed than qt.