BUG/MEDIUM: mux-spop: Respect the negociated max-frame-size value to send frames

BUG/MEDIUM: mux-spop: Respect the negociated max-frame-size value to send frames

When a SPOP connection is opened, the maximum size for frames is negociated.
This negociated size is properly used when a frame is received and if a too
big frame is detected, an error is triggered. However, the same was not
performed on the sending path. No check was performed on frames sent to the
agent. So it was possible to send frames bigger than the maximum size
supported by the the SPOE agent.

Now, the size of NOTIFY and DISCONNECT frames is checked before sending them
to the agent.

Thanks to Miroslav to have reported the issue.

This patch must be backported to 3.1.