MEDIUM: mux-fcgi: Introduce flags to deal with connection read/write errors

MEDIUM: mux-fcgi: Introduce flags to deal with connection read/write errors

Similarly to the H1 and H2 multiplexers, FCFI_CF_ERR_PENDING is now used to
report an error when we try to send data and FCGI_CF_ERROR to report an
error when we try to read data. In other funcions, we rely on these flags
instead of connection ones. Only FCGI_CF_ERROR is considered as a final
error.  FCGI_CF_ERR_PENDING does not block receive attempt.

In addition, FCGI_CF_EOS flag was added. we rely on it to test if a read0
was received or not.