lavf/tcp: add option to setting Maximum Segment Size

This can change the the MSS value announced to the other end in
the initial TCP packet, it's can be used when failed Path MTU
discovery.
Signed-off-by: Jun Zhao <mypopydev@gmail.com>

libavformat/tcp.c

@@ -42,6 +42,9 @@ typedef struct TCPContext {
     int recv_buffer_size;
     int send_buffer_size;
     int tcp_nodelay;
+#if !HAVE_WINSOCK2_H
+    int tcp_mss;
+#endif /* !HAVE_WINSOCK2_H */
 } TCPContext;
 
 #define OFFSET(x) offsetof(TCPContext, x)
@@ -54,6 +57,9 @@ static const AVOption options[] = {
     { "send_buffer_size", "Socket send buffer size (in bytes)",                OFFSET(send_buffer_size), AV_OPT_TYPE_INT, { .i64 = -1 },         -1, INT_MAX, .flags = D|E },
     { "recv_buffer_size", "Socket receive buffer size (in bytes)",             OFFSET(recv_buffer_size), AV_OPT_TYPE_INT, { .i64 = -1 },         -1, INT_MAX, .flags = D|E },
     { "tcp_nodelay", "Use TCP_NODELAY to disable nagle's algorithm",           OFFSET(tcp_nodelay), AV_OPT_TYPE_BOOL, { .i64 = 0 },             0, 1, .flags = D|E },
+#if !HAVE_WINSOCK2_H
+    { "tcp_mss",     "Maximum segment size for outgoing TCP packets",          OFFSET(tcp_mss),     AV_OPT_TYPE_INT, { .i64 = -1 },         -1, INT_MAX, .flags = D|E },
+#endif /* !HAVE_WINSOCK2_H */
     { NULL }
 };
 
@@ -153,6 +159,11 @@ static int tcp_open(URLContext *h, const char *uri, int flags)
     if (s->tcp_nodelay > 0) {
         setsockopt (fd, IPPROTO_TCP, TCP_NODELAY, &s->tcp_nodelay, sizeof (s->tcp_nodelay));
     }
+#if !HAVE_WINSOCK2_H
+    if (s->tcp_mss > 0) {
+        setsockopt (fd, IPPROTO_TCP, TCP_MAXSEG, &s->tcp_mss, sizeof (s->tcp_mss));
+    }
+#endif /* !HAVE_WINSOCK2_H */
 
     if (s->listen == 2) {
         // multi-client