MINOR: mux-fcgi: Use flags of HTX blocks to detect end of message during sending

MINOR: mux-fcgi: Use flags of HTX blocks to detect end of message during sending

When a message is sent, the end of the message is now detected by testing
flags of each HTX blocs. Otherwise, the logic remains mostly unchanged. The
way the end of the message is handled in fcgi_strm_send_stdin() was modified
a bit but the result is the same. Now, we rely on a variable to decide, at
the end, if an empty STDIN record should be sent or not. This avoid an extra
test on the HTX message to be sure it is empty.