Clean up minor bugs and issues in util/network.c

Fix compilation without IPv6 support and fix several comments.
Use the same warning message format in network_bind_ipv4 and
network_bind_ipv6.

network_connect, network_connect_host, and network_client_create now
accept "any" as a source as well as "all", as a synonym for letting
the operating system pick, for parallelism with network_bind_ipv4 and
network_bind_ipv6.