prevent redundant notifications from /serialosc/notify

[brian crabtree]

presently there are some situations where serialoscd can accumulate the same host/port many times in its notification queue

• run serialosc client application, which sends /notify, gets on queue
• quit
• run same application, which sends /notify again
• plug device, two /add messages will show up

fix would be to check the queue for existing host/port match before adding it again:

https://github.com/monome/serialosc/blob/main/src/serialoscd/uv.c#L389

the VECTOR module does not seem to have a PEEK function, however.