Implemented and exposed rejection reason

[ethouris]

The rejection reason has been implemented. As a demonstration, also reported by srt-test-live.

Internal changes: the codes for m_iReqType in the handshake contain error values that contain reject reason, which occupy all values above 1000 (as previously erroneous values). Backward compatibility assignments to codes 1002 and 1004 as SRT_REJ_PEER and SRT_REJ_ROGUE respectively, however when getting the rejection reason from the peer in older version than this PR will get always SRT_REJ_PEER code, only newer version will report correct detailed code.

API changes: new API entries:

srt_getrejectreason(SRTSOCKET): call this function immediately after the failure has reported the error code as SRT_ECONNREJ. This gets the reject reason as a numeric value, however potentially with the SRT_REJ_PEERREP bit set, which means that this code has been reported by peer, not by agent.

srt_rejectreason_str(enum SRT_REJECT_REASON): will return the message code, disregarding the SRT_REJ_PEERREP bit - this one must be checked separately (this is a result from the simplicity of the implementation, where the code is mapped to a message in an array).

Fixes #684

[ethouris]

What I'm not sure about, after time, is that whether this "peer reported" flag is really required. This complicates the API usage for the user, while for the user it usually doesn't matter if the rejection was initiated by the listener or the caller, moreover, usually it will be the listener that rejected the connection, while this can only be reported on the caller. I'm thinking of removing this bit completely as unnecessary.

[maxsharabayko]

Consider renaming the SRT_REJ_STRICTENC, given that strict encryption is also to be renamed (#755).
Maybe SRT_REJ_BADENCRYPT?

[maxsharabayko]

Usage example:

SRTSOCKET sock = srt_create_socket();
struct sockaddr_in sa;    // Fill in the structure

const int connect_res = srt_connect(sock, (struct sockaddr*)&sa, sizeof sa); 
if (connect_res == SRT_ERROR)
{
	const int error_code = srt_getlasterror();
	if (error_code == SRT_ECONNREJ)
	{
		const SRT_REJECT_REASON reason = srt_getrejectreason(m_sock);
		const char* reason_str = srt_rejectreason_str(reason); 
		std::cerr << "Connection rejected: " << reason_str << "\n";
	}
}

[ethouris]

Consider renaming the SRT_REJ_STRICTENC, given that strict encryption is also to be renamed (#755).
Maybe SRT_REJ_BADENCRYPT?

I think SRT_REJ_UNSECURE is good enough.