Add -timelimit option

This option limits the CPU time used by ffmpeg to the number of seconds
specified.  After this time, the OS sends a SIGXCPU signal, which we
handle and attempt to exit cleanly.  If the process is stuck, the OS
will deliver a SIGKILL one second later, forcibly terminating the
process.

This functionality is useful in automated setups where a runaway ffmpeg
process would otherwise go undetected.

Originally committed as revision 21347 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -353,6 +353,10 @@ static void term_init(void)
 
     signal(SIGINT , sigterm_handler); /* Interrupt (ANSI).  */
     signal(SIGTERM, sigterm_handler); /* Termination (ANSI).  */
+#ifdef SIGXCPU
+    signal(SIGXCPU, sigterm_handler);
+#endif
+
     /*
     register a function to be called at normal program termination
     */
@@ -3863,6 +3867,7 @@ static const OptionDef options[] = {
     { "dframes", OPT_INT | HAS_ARG, {(void*)&max_frames[CODEC_TYPE_DATA]}, "set the number of data frames to record", "number" },
     { "benchmark", OPT_BOOL | OPT_EXPERT, {(void*)&do_benchmark},
       "add timings for benchmarking" },
+    { "timelimit", OPT_FUNC2 | HAS_ARG, {(void*)opt_timelimit}, "set max runtime in seconds", "limit" },
     { "dump", OPT_BOOL | OPT_EXPERT, {(void*)&do_pkt_dump},
       "dump each input packet" },
     { "hex", OPT_BOOL | OPT_EXPERT, {(void*)&do_hex_dump},