The rtmp protocol uses nonblocking reads, to poll for incoming
messages from the server while publishing a stream.

Prior to 94599a6d and
d13b124e, the tls protocol
handled the nonblocking flag, mostly as a side effect from not
using custom IO callbacks for reading from the socket. When custom
IO callbacks were taken into use in
d15eec4d, the handling of a nonblocking
socket wasn't necessary for the default blocking mode any longer.

The code was simplified, since it was overlooked that other code
within libavformat actually used the tls protocol in nonblocking mode.

This fixes publishing over rtmps, with the gnutls backend.
Signed-off-by: Martin Storsjö <martin@martin.st>