BUG/MEDIUM: stream: don't consider abortonclose on muxes which close cleanly

BUG/MEDIUM: stream: don't consider abortonclose on muxes which close cleanly

The H2 mux can cleanly report an error when a client closes, which is not
the case for the pass-through mux which only reports shutr. That was the
reason why "option abortonclose" was created since there was no way to
distinguish a clean shutdown after sending the request from an abort.

The problem is that in case of H2, the streams are always shut read after
the request is complete (when the END_STREAM flag is received), and that
when this lands on a backend configured with "option abortonclose", this
aborts the request. Disabling abortonclose is not always an option when
H1 and H2 have to coexist.

This patch makes use of the newly introduced mux capabilities reported
via the stream interface's SI_FL_CLEAN_ABRT indicating that the mux is
safe and that there is no need to turn a clean shutread into an abort.
This way abortonclose has no effect on requests initiated from an H2
mux.

This patch as well as these 3 previous ones need to be backported to
1.8 :
 - BUG/MINOR: h2: properly report a stream error on RST_STREAM
 - MINOR: mux: add flags to describe a mux's capabilities
 - MINOR: stream-int: set flag SI_FL_CLEAN_ABRT when mux supports clean aborts