srt-live-transmit: add option to cancel timeout

[zmousm]

Add an option (-taoc, -timeout-abort-on-connect) to cancel the timeout (as set with -t) after a client is accepted. This allows for srt-live-transmit to run in listener mode and wait for a limited amount of time (timeout) for a client to connect.

Due to the paired tx/rx operation of srt-live-transmit and the fact that a sender process may not accept multiple clients, should one want to operate such a server they would need to devise a way to spawn sender processes as necessary. In such a case sender processes spawned in listener mode wait for the client to connect; it then becomes important to enforce a timeout so the sender process does not wait forever. You already have the timeout option for srt-live-transmit, but that works as an unconditional exit timer. This patch introduces a second option, which changes the behavior of timeout: once a client is accepted the pending alarm(2) is canceled.

This is probably an abuse of the timeout option, but it seems to be the only way to achieve a listener timeout. The timeout (SRTO_RCVTIMEO, SRTO_SNDTIMEO) and conntimeo parameters do not seem to have any effect at the particular stage.

[zmousm]

[...] should one want to operate such a server they would need to devise a way to spawn sender processes as necessary

I wrote a CGI script as a simple implementation for that.