MEDIUM: mux-fcgi: Add a function to propagate termination flags from fstrm to SE

MEDIUM: mux-fcgi: Add a function to propagate termination flags from fstrm to SE

The function fcgi_strm_propagate_term_flags() was added to check the FSTRM
state and evaluate when EOI/EOS/ERR_PENDING/ERROR flags must be set on the
SE. It is not the only place where those flags are set. But it centralizes
the synchro between the FCGI stream and the SC.

For now, this function is only used at the end of fcgi_rcv_buf(). But it
will be used to fix a potential bug.