parse-helpers: allow port 0 for socket bind items

parse-helpers: allow port 0 for socket bind items

This patch adds a new parameter to parse_ip_port_range, giving callers
the option to allow ranges to have their min be 0 instead of 1.

This is then used by parse_ip_ports_token, intern used by
parse_socket_bind_item to allow port 0 when restricting bind system
calls with SocketBindDeny / SocketBindAllow.

With this, users running server software written using the golang
standard library will be able to effectively sandbox their software,
albeit with a small loss in security protections by allowing the
process to bind on a random port in the
/proc/sys/net/ipv4/ip_local_port_range.