Add tcp_log_addrs() function to generate and standardized TCP log line
for use thoughout the tcp subsystem.

It is IPv4 and IPv6 aware creates a line in the following format:

"TCP: [1.2.3.4]:50332 to [1.2.3.4]:80 tcpflags <RST>"

A "\n" is not included at the end. The caller is supposed to add
further information after the standard tcp log header.

The function returns a NUL terminated string which the caller has
to free(s, M_TCPLOG) after use. All memory allocation is done
with M_NOWAIT and the return value may be NULL in memory shortage
situations.

Either struct in_conninfo || (struct tcphdr && (struct ip || struct
ip6_hdr) have to be supplied.

Due to ip[6].h header inclusion limitations and ordering issues the
struct ip and struct ip6_hdr parameters have to be casted and passed
as void * pointers.

tcp_log_addrs(struct in_conninfo *inc, struct tcphdr *th, void *ip4hdr,

void *ip6hdr)

Usage example:

struct ip *ip;
char *tcplog;

if (tcplog = tcp_log_addrs(NULL, th, (void *)ip, NULL)) {
log(LOG_DEBUG, "%s; %s: Connection attempt to closed port\n",

	    tcplog, __func__);

free(s, M_TCPLOG);
}