make webpush use our notification server vkey instead of a random, ill-formated

key (necessary for some client notifications, such as moshidon)

doc/snac.8

@@ -294,6 +294,8 @@ The URL to a favicon. If it's not set, the default one is used instead.
 The maximum number of days an instance is considered down after continuous failed
 outgoing messages (default: 15). Anyway, whenever any incoming activity from a
 failed instance is detected, this counter is reset for it.
+.It Ic vkey
+Public vapid key. Used for notification on some client.
 .El
 .Pp
 You must restart the server to make effective these changes.

mastoapi.c

@@ -3044,7 +3044,10 @@ int mastoapi_post_handler(const xs_dict *req, const char *q_path,
             xs *app  = xs_dict_new();
             xs *id   = xs_replace_i(tid(0), ".", "");
             xs *csec = random_str();
-            xs *vkey = random_str();
+            char *vkey = (char *)xs_dict_get(srv_config, "vkey");
+            if (vkey == NULL)
+                vkey = random_str();
+
             xs *cid  = NULL;
 
             /* pick a non-existent random cid */