BUG/MEDIUM: mux-fcgi: Handle cases where the HTX EOM block cannot be inserted

BUG/MEDIUM: mux-fcgi: Handle cases where the HTX EOM block cannot be inserted

During the HTTP response parsing, if there is not enough space in the channel's
buffer, it is possible to fail to add the HTX EOM block while all data in the
rxbuf were consumed. As for the h1 mux, we must notify the conn-stream the
buffer is full to have a chance to add the HTX EOM block later. In this case, we
must also be carefull to not report a server abort by setting too early the
CS_FL_EOS flag on the conn-stream.

To do so, the FCGI_SF_APPEND_EOM flag must be set on the FCGI stream to know the
HTX EOM block is missing.

This patch must be backported to 2.1.