MINOR: mux-spop: Report EOI on the SE when a ACK is received for a stream

MINOR: mux-spop: Report EOI on the SE when a ACK is received for a stream

The spop stream now reports the end of input when the ACK is transferred to
the SPOE applet. To do so, the flag SPOP_SF_ACK_RCVD was added. It is set on
the SPOP stream when its ACK is received by the SPOP connection.

In addition when SPOP stream flags are propagated to the SE, the error is
now reported if end of input was not reached instead of testing the
connection error code. It is more accurate.

This patch should be backported to 3.1.