Add 'timeout' option to UDP protocol

This patch accepts 'timeout' option for input mode only. As far as i know, UDP output cannot introduce delays.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/protocols.texi

@@ -694,6 +694,9 @@ packets with size of 188 bytes. If not specified defaults to 7*4096.
 @item overrun_nonfatal=@var{1|0}
 Survive in case of UDP receiving circular buffer overrun. Default
 value is 0.
+
+@item timeout=@var{microseconds}
+In read mode: if no data arrived in more than this time interval, raise error.
 @end table
 
 Some usage examples of the UDP protocol with @command{ffmpeg} follow.

libavformat/udp.c

@@ -549,6 +549,8 @@ static int udp_open(URLContext *h, const char *uri, int flags)
                     break;
             }
         }
+        if (!is_output && av_find_info_tag(buf, sizeof(buf), "timeout", p))
+            h->rw_timeout = strtol(buf, NULL, 10);
     }
 
     /* fill the dest addr */