* p2p: add low port check in dialer

We already have a check like this for UDP ports, add a similar one in
the dialer. This prevents dials to port zero and it's also an extra
layer of protection against spamming HTTP servers.

* p2p/discover: use errLowPort in v4 code

* p2p: change port check

* p2p: add comment

* p2p/simulations/adapters: ensure assigned port is in all node records