MEDIUM: mux-h1: Clarify how shutr/shutw are handled

MEDIUM: mux-h1: Clarify how shutr/shutw are handled

Now, h1_shutr() only do a shutdown read and try to set the flag
H1C_F_CS_SHUTDOWN if shutdown write was already performed. On its side,
h1_shutw(), if all conditions are met, do the same for the shutdown write. The
real connection close is done when the mux h1 is released, in h1_release().

The flag H1C_F_CS_SHUTW was renamed to H1C_F_CS_SHUTDOWN to be less ambiguous.

This patch may be backported to 1.9.