MINOR: mux-h1: Use flags of HTX blocks to detect end of message during sending

MINOR: mux-h1: 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. A
test in h1_make_eoh() was slightly changed but it should be harmless.
Especially because HTX_BLK_FL_EOM cannot be set on the EOH block if it is
followed by something else than tunneled data, which is unexpected and
invalid in that case. All conditions to establish a tunnel at this stage
were already evaluated. And if any, the error will be detected in
h1_make_trailers() function.