watch: can use .magnet files

Based on:
* https://trac.transmissionbt.com/ticket/4710#comment:18
* https://forum.transmissionbt.com/viewtopic.php?f=3&t=13377#post_content73143

daemon/daemon.c

@@ -201,14 +201,28 @@ static tr_watchdir_status onFileAdded(tr_watchdir_t dir, char const* name, void*
 {
     tr_session* session = context;
 
-    if (!tr_str_has_suffix(name, ".torrent"))
+    if (!tr_str_has_suffix(name, ".torrent") && !tr_str_has_suffix(name, ".magnet"))
     {
         return TR_WATCHDIR_IGNORE;
     }
 
     char* filename = tr_buildPath(tr_watchdir_get_path(dir), name, NULL);
     tr_ctor* ctor = tr_ctorNew(session);
-    int err = tr_ctorSetMetainfoFromFile(ctor, filename);
+    int err;
+    if (tr_str_has_suffix(name, ".torrent"))
+    {
+        err = tr_ctorSetMetainfoFromFile(ctor, filename);
+    }
+    else
+    {
+        size_t len;
+        uint8_t* link = tr_loadFile(filename, &len, NULL);
+        if (link != NULL)
+        {
+            err = tr_ctorSetMetainfoFromMagnetLink(ctor, link);
+            tr_free(link);
+        }
+    }
 
     if (err == 0)
     {