MINOR: mux-h1: Add a dedicated enum to deal with H1 connection state

MINOR: mux-h1: Add a dedicated enum to deal with H1 connection state

The H1 connection state will be handled is a dedicated field. To do so,
h1_cs enum was added. The different states are more or less equivalent to
H1C_F_ST_* flags:

 * H1_CS_IDLE      <=> H1C_F_ST_IDLE
 * H1_CS_EMBRYONIC <=> H1C_F_ST_EMBRYONIC
 * H1_CS_UPGRADING <=> H1C_F_ST_ATTACHED && !H1C_F_ST_READY
 * H1_CS_RUNNING   <=> H1C_F_ST_ATTACHED && H1C_F_ST_READY
 * H1_CS_CLOSING   <=> H1C_F_ST_SHUTDOWN && (H1C_F_ST_ATTACHED || b_data(&h1c->ibuf))
 * H1_CS_CLOSED    <=> H1C_F_ST_SHUTDOWN && !H1C_F_ST_ATTACHED && !b_data(&h1c->ibuf)

In addition, in this patch, the h1_is_alive() and h1_close() function are
added. The first one will be used to know if a H1 connection is alive or
not. The second one will be used to set the connection in CLOSING or CLOSED
state, depending on the output buffer state and if there is still a H1
stream or not.

For now, the H1 connection state is not used.