basic udisks2 support

To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices.  Later, this
will allow mounting removable media with a new storage plugin.

src/CommandLine.cxx

@@ -226,6 +226,9 @@ static void version(void)
 #ifdef ENABLE_DBUS
 	       " dbus"
 #endif
+#ifdef ENABLE_UDISKS
+	       " udisks"
+#endif
 #ifdef USE_EPOLL
 	       " epoll"
 #endif