linux,ssh: set IP_BIND_ADDRESS_NO_PORT on connected socket

if binding to a local address (ssh -b ...) the source port is taken
at bind() time when the kernel does not know if the socket
will be connect()ed or listen()ed on. It also does not
know the destination host or port so it has to reserve the port
until the socket is closed, effectively limiting the number
of useful source ports to ~32k. a very small number in the modern era.

Set IP_BIND_ADDRESS_NO_PORT, which delays source port allocation
to connect() time allowing a few million connections out from the
same -b address.

sshconnect.c

@@ -415,6 +415,9 @@ ssh_create_socket(struct addrinfo *ai)
 		error_f("getnameinfo failed: %s", ssh_gai_strerror(r));
 		goto fail;
 	}
+#ifdef IP_BIND_ADDRESS_NO_PORT
+	(void) setsockopt(sock, SOL_IP, IP_BIND_ADDRESS_NO_PORT, &(int) {1}, sizeof(int));
+#endif
 	if (bind(sock, (struct sockaddr *)&bindaddr, bindaddrlen) != 0) {
 		error("bind %s: %s", ntop, strerror(errno));
 		goto fail;