MINOR: tcp: socket translate TCP_KEEPIDLE for macOs equivalent

On Linux the interval before starting to send TCP keep-alive packets
is defined by TCP_KEEPIDLE. MacOS has an equivalent with TCP_KEEPIDLE,
which also uses seconds as a unit, so it's possible to simply remap the
definition of TCP_KEEPIDLE to TCP_KEEPALIVE there and get it to seamlessly
work. The other settings (interval and count) are not present, though.

include/haproxy/compat.h

@@ -284,6 +284,7 @@ typedef struct { } empty_t;
 #include <malloc/malloc.h>
 #define malloc_usable_size malloc_size
 #define HA_HAVE_MALLOC_ZONE
+#define TCP_KEEPIDLE TCP_KEEPALIVE
 #define TCP_INFO TCP_CONNECTION_INFO
 #define tcp_info tcp_connection_info
 #endif