MEDIUM: listener/mux-h2: implement idle-ping on frontend side

MEDIUM: listener/mux-h2: implement idle-ping on frontend side

This commit is the counterpart of the previous one, adapted on the
frontend side. "idle-ping" is added as keyword to bind lines, to be able
to refresh client timeout of idle frontend connections.

H2 MUX behavior remains similar as the previous patch. The only
significant change is in h2c_update_timeout(), as idle-ping is now taken
into account also for frontend connection. The calculated value is
compared with http-request/http-keep-alive timeout value. The shorter
delay is then used as expired date. As hr/ka timeout are based on
idle_start, this allows to run them in parallel with an idle-ping timer.