[FR] Reason for closing a connection

[maxsharabayko]

A connection can be closed by an idle timeout or due to a reason.

1. API has to be extended to expose the reason why a socket was closed, e.g. int srt_brokenreason(SRTSOCKET s).
2. The SHUTDOWN control packet has to be extended to allow reporting the reason for closing the connection. The reason may be SRT-internal one, or initiated by the application.
3. If a connection is closed by the application, it can optionally provide the reason for closing it. Then the srt_close API function needs to be accompanied with srt_close_ext(SRTSOCKET s, int reason).

See also #478.