anet.c: use SO_REUSEADDR when creating listening sockets.

It used to be ok, but the socket option was removed when adding IPv6
support.
anet.c: save some vertical space.
Sentinel: make sure published addr/id buffer is large enough.

With ipv6 support we need more space, so we account for the IP address
max size plus what we need for the Run ID, port, flags.