Remove REUSEABLE_PORT flag from tcp_listener_bind

[jaromil]

After extensive testing we noticed the REUSABLE_PORT flag really breaks most ARM builds: for some reason Linux kernels built on ARM do not support them. This issue was introduced in 79f3ad9 and is addressed already in various commits: 0a0160b and d1c1916 and 8ff4d77.

Removing the flag alltogether solves the issue, while I'm not entirely sure the initial intention of "allowing round-robining to multiple dnscrypt-proxy daemons on a single port" is really enabled by it.

We use the following diff in Dowse as a fix and it may be worth considering mergin, unless anyone strongly relies on this RR feature:

diff --git a/src/proxy/tcp_request.c b/src/proxy/tcp_request.c
index 7ad3efc..cdb6a69 100644
--- a/src/proxy/tcp_request.c
+++ b/src/proxy/tcp_request.c
@@ -560,7 +560,7 @@ tcp_listener_bind(ProxyContext * const proxy_context)
 #endif
     if (proxy_context->tcp_listener_handle == -1) {
         unsigned int flags = LEV_OPT_CLOSE_ON_FREE | LEV_OPT_CLOSE_ON_EXEC |
-                             LEV_OPT_REUSEABLE | LEV_OPT_REUSEABLE_PORT |
+                             LEV_OPT_REUSEABLE |
                              LEV_OPT_DEFERRED_ACCEPT;
         for (;;) {
             proxy_context->tcp_conn_listener =
diff --git a/src/proxy/udp_request.c b/src/proxy/udp_request.c
index 455e907..a14822e 100644
--- a/src/proxy/udp_request.c
+++ b/src/proxy/udp_request.c
@@ -492,7 +492,6 @@ udp_listener_kill_oldest_request(ProxyContext * const proxy_context)
 int
 udp_listener_bind(ProxyContext * const proxy_context)
 {
-    int optval = 1;
     if (proxy_context->udp_listener_handle == -1) {
         if ((proxy_context->udp_listener_handle = socket
              (proxy_context->local_sockaddr.ss_family,
@@ -501,9 +500,6 @@ udp_listener_bind(ProxyContext * const proxy_context)
                             "Unable to create a socket (UDP)");
             return -1;
         }
-#if defined(__linux__) && defined(SO_REUSEPORT) && !defined(NO_REUSEPORT)
-        setsockopt(proxy_context->udp_listener_handle, SOL_SOCKET, SO_REUSEPORT, &optval, sizeof(optval));
-#endif
         if (bind(proxy_context->udp_listener_handle,
                  (struct sockaddr *) &proxy_context->local_sockaddr,
                  proxy_context->local_sockaddr_len) != 0) {