Rework the "SetUsername" functionality

- Remove SetUsername itself, and add a username parameter to DeviceClaim,
  ListEnrolledFingers and DeleteEnrolledFingers.
- For each of those calls, check that the incoming connection is allowed
  to operate on that particular username
- Don't require a claimed device to list or remove fingerprints
- Clean up username and sender when releasing the device
- Modify the storage backend to not require an opened device to list
  or delete fingerprints
- Add a simple test program to list registered fingerprints for the
  usernames passed as argument

src/device.xml

@@ -4,20 +4,23 @@
 		<annotation name="org.freedesktop.DBus.GLib.CSymbol"
 			value="fprint_device" />
 
-		<method name="Claim">
-			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
-		</method>
-
-		<method name="Release">
-			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
-		</method>
-
 		<method name="ListEnrolledFingers">
+			<arg type="s" name="username" direction="in" />
 			<arg type="au" name="enrolled_fingers" direction="out" />
 			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
 		</method>
 
 		<method name="DeleteEnrolledFingers">
+			<arg type="s" name="username" direction="in" />
+			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
+		</method>
+
+		<method name="Claim">
+			<arg type="s" name="username" direction="in" />
+			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
+		</method>
+
+		<method name="Release">
 			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
 		</method>
 
@@ -47,11 +50,6 @@
 			<arg type="i" name="result" />
 		</signal>
 
-		<method name="SetUsername">
-			<arg type="s" name="username" direction="in" />
-			<annotation name="org.freedesktop.DBus.GLib.Async" value="" />
-		</method>
-
 	</interface>
 </node>