REORG: tproxy: prepare the transparent proxy defines for accepting other OSes

This patch does not change the logic of the code, it only changes the
way OS-specific defines are tested.

At the moment the transparent proxy code heavily depends on Linux-specific
defines. This first patch introduces a new define "CONFIG_HAP_TRANSPARENT"
which is set every time the defines used by transparent proxy are present.
This also means that with an up-to-date libc, it should not be necessary
anymore to force CONFIG_HAP_LINUX_TPROXY during the build, as the flags
will automatically be detected.

The CTTPROXY flags still remain separate because this older API doesn't
work the same way.

A new line has been added in the version output for haproxy -vv to indicate
what transparent proxy support is available.
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 97bc33b..f59c23b 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -122,15 +122,24 @@
 	struct sockaddr_storage bind_addr;
 	int foreign_ok = 0;
 	int ret;
-
-#ifdef CONFIG_HAP_LINUX_TPROXY
 	static int ip_transp_working = 1;
 	static int ip6_transp_working = 1;
+
 	switch (local->ss_family) {
 	case AF_INET:
 		if (flags && ip_transp_working) {
-			if (setsockopt(fd, SOL_IP, IP_TRANSPARENT, &one, sizeof(one)) == 0
-			    || setsockopt(fd, SOL_IP, IP_FREEBIND, &one, sizeof(one)) == 0)
+			/* This deserves some explanation. Some platforms will support
+			 * multiple combinations of certain methods, so we try the
+			 * supported ones until one succeeds.
+			 */
+			if (0
+#if defined(IP_TRANSPARENT)
+			    || (setsockopt(fd, SOL_IP, IP_TRANSPARENT, &one, sizeof(one)) == 0)
+#endif
+#if defined(IP_FREEBIND)
+			    || (setsockopt(fd, SOL_IP, IP_FREEBIND, &one, sizeof(one)) == 0)
+#endif
+			    )
 				foreign_ok = 1;
 			else
 				ip_transp_working = 0;
@@ -138,14 +147,18 @@
 		break;
 	case AF_INET6:
 		if (flags && ip6_transp_working) {
-			if (setsockopt(fd, SOL_IPV6, IPV6_TRANSPARENT, &one, sizeof(one)) == 0)
+			if (0
+#if defined(IPV6_TRANSPARENT)
+			    || (setsockopt(fd, SOL_IPV6, IPV6_TRANSPARENT, &one, sizeof(one)) == 0)
+#endif
+			    )
 				foreign_ok = 1;
 			else
 				ip6_transp_working = 0;
 		}
 		break;
 	}
-#endif
+
 	if (flags) {
 		memset(&bind_addr, 0, sizeof(bind_addr));
 		bind_addr.ss_family = remote->ss_family;
@@ -621,25 +634,35 @@
 	if (!ext)
 		setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &one, sizeof(one));
 #endif
-#ifdef CONFIG_HAP_LINUX_TPROXY
+
 	if (!ext && (listener->options & LI_O_FOREIGN)) {
 		switch (listener->addr.ss_family) {
 		case AF_INET:
-			if ((setsockopt(fd, SOL_IP, IP_TRANSPARENT, &one, sizeof(one)) == -1)
-			    && (setsockopt(fd, SOL_IP, IP_FREEBIND, &one, sizeof(one)) == -1)) {
+			if (1
+#if defined(IP_TRANSPARENT)
+			    && (setsockopt(fd, SOL_IP, IP_TRANSPARENT, &one, sizeof(one)) == -1)
+#endif
+#if defined(IP_FREEBIND)
+			    && (setsockopt(fd, SOL_IP, IP_FREEBIND, &one, sizeof(one)) == -1)
+#endif
+			    ) {
 				msg = "cannot make listening socket transparent";
 				err |= ERR_ALERT;
 			}
 		break;
 		case AF_INET6:
-			if (setsockopt(fd, SOL_IPV6, IPV6_TRANSPARENT, &one, sizeof(one)) == -1) {
+			if (1
+#if defined(IPV6_TRANSPARENT)
+			    && (setsockopt(fd, SOL_IPV6, IPV6_TRANSPARENT, &one, sizeof(one)) == -1)
+#endif
+			    ) {
 				msg = "cannot make listening socket transparent";
 				err |= ERR_ALERT;
 			}
 		break;
 		}
 	}
-#endif
+
 #ifdef SO_BINDTODEVICE
 	/* Note: this might fail if not CAP_NET_RAW */
 	if (!ext && listener->interface) {
@@ -1546,7 +1569,7 @@
 }
 #endif
 
-#ifdef CONFIG_HAP_LINUX_TPROXY
+#ifdef CONFIG_HAP_TRANSPARENT
 /* parse the "transparent" bind keyword */
 static int bind_parse_transparent(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
@@ -1695,7 +1718,7 @@
 #ifdef TCP_FASTOPEN
 	{ "tfo",           bind_parse_tfo,          0 }, /* enable TCP_FASTOPEN of listening socket */
 #endif
-#ifdef CONFIG_HAP_LINUX_TPROXY
+#ifdef CONFIG_HAP_TRANSPARENT
 	{ "transparent",   bind_parse_transparent,  0 }, /* transparently bind to the specified addresses */
 #endif
 #ifdef IPV6_V6ONLY