obs-ffmpeg: Fix reconnection for SRT protocol

This is the first part of a fix for reconnection issues with SRT
protocol.
This part implements the logic on the side of the protocol.
The fixes are as follows:
- a cleanup of return values of the various functions used. The main
libsrt_open function returns OBS_OUTPUT_XXX enum members. But the other
functions return now only AVERROR(xxx).
- an explicit check of the socket state before returning
OBS_OUTPUT_SUCCESS. This fixes an issue due to relying on socket
non-blocking mode; in caller mode, the srt_connect function will return
a success even before a connection has succeeded since it relies on
notifications by srt_epoll_wait. This would prompt obs UI to display a
successful reconnection in spite of a failure.

Signed-off-by: pkv <pkv@obsproject.com>
This commit is contained in:
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