dom0-updates: wait for apper to finish, then remove "updates pending" flag

Unfortunate side effect (to be solved at Qubes Manager side): it will
block Qubes Manager until all updates are installed (not only updater
launched).

Fixes QubesOS/qubes-issues#1511

(cherry picked from commit cb6a52a652)

dom0-updates/qubes-dom0-update

@@ -86,7 +86,7 @@ if [ "$GUI" == "1" ]; then
         if type $app &>/dev/null; then
             guiapp=$app
             case $guiapp in
-                apper) guiapp="apper --updates" ;;
+                apper) guiapp="apper --updates --nofork" ;;
                 *) guiapp=$app ;;
             esac
             break;