importd: when listing transfers, show progress percentage

With this change the pull protocol implementation processes will pass
progress data to importd which then passes this information on via the
bus. We use sd_notify() as generic transport for this communication,
making importd listen to them, while matching the incoming messages to
the right transfer.

src/shared/def.h

@@ -81,3 +81,6 @@
 #ifndef TTY_GID
 #define TTY_GID 5
 #endif
+
+#define NOTIFY_FD_MAX 768
+#define NOTIFY_BUFFER_MAX PIPE_BUF