MINOR: stconn: Add a flag to report EOS at the stream-connector level

MINOR: stconn: Add a flag to report EOS at the stream-connector level

SC_FL_EOS flag is added to report the end-of-stream at the SC level. It will
be used to distinguish end of stream reported by the endoint, via the
SE_FL_EOS flag, and the abort triggered by the stream, via the
SC_FL_ABRT_DONE flag.

In this patch, the flag is defined and is systematically tested everywhere
SC_FL_ABRT_DONE is tested. It should be safe because it is never set.