core: add chaining helper functions to callback info

Helpful when chaining a number of commands together when you want
to schedule the callback info only after all of them complete.

src/mm-callback-info.h

@@ -25,6 +25,9 @@ typedef void (*MMCallbackInfoInvokeFn) (MMCallbackInfo *info);
 struct _MMCallbackInfo {
     guint32 refcount;
 
+    /* # of ops left in this callback chain */
+    guint32 chain_left;
+
     GData *qdata;
     MMModem *modem;
 
@@ -70,5 +73,8 @@ gpointer        mm_callback_info_get_data (MMCallbackInfo *info,
 MMCallbackInfo *mm_callback_info_ref (MMCallbackInfo *info);
 void            mm_callback_info_unref (MMCallbackInfo *info);
 
+void            mm_callback_info_chain_start (MMCallbackInfo *info, guint num);
+void            mm_callback_info_chain_complete_one (MMCallbackInfo *info);
+
 #endif /* MM_CALLBACK_INFO_H */