Rewrite log_peer_info() and log_peer_warn[x]()

Rewrite log_peer_info() and log_peer_warn[x]()

Passing the peer description as part of the format string was a bad idea
since the peer description may include some % signs (e.g. for link local
IPv6 addresses). So instead of asprintf a new fmt string use vasprintf
to get the message and then use logit("%s: %s", peer_info, msg).
OK tb@