BUG/MEDIUM: mux_h1: Don't call h1_send if we subscribed().

BUG/MEDIUM: mux_h1: Don't call h1_send if we subscribed().

In h1_snd_buf(), only attempt to call h1_send() if we haven't
already subscribed.
It makes no sense to do it if we subscribed, as we know we failed
to send before, and will create a useless call to sendto(), and
in 2.2, the call to raw_sock_from_buf() will disable polling if
it is enabled.

This should be backported to 2.2, 2.1, 2.0 and 1.9.