[FeatureRequest] Reason for rejected connection

[maxsharabayko]

There needs to be a way to find out the reason for the connection being rejected.

Currently in blocking mode connect() returns -1, and the error code returned by srt_getlasterror() is Connection setup failure: connection rejected (code 1002). But the reason is unclear, as this is returned for several reasons.
In non-blocking mode the error after srt_epoll_wait() on the caller socket returns -1, is
Non-blocking call failure: transmission timed out (code 6003)
That does not give any clue either.

There is MN_SECURITY minor error code, but it is disabled for backward compatibility reasons.

For the listening socket would be good to have a way to notify about the failing connections. A socket option with a callback function pointer?

[maxsharabayko]

PR #720