MINOR: session: define flag to explicitely release listener on free

MINOR: session: define flag to explicitely release listener on free

When a session is allocated for a FE connection, session_free() is
responsible to call listener_release() to decrement listener connection
counters and resume listening.

Until now, <listener> member of session was tested inside session_free()
before invocating listener_release(). To highlight more explicitely the
relation between sessions and listeners, introduce a new flag
SESS_FL_RELEASE_LI. Only session with such flag set will invoke
listener_release() on their cleanup. Flag is set inside
session_accept_fd() on success.

This patch has no functional change. However, it will be useful to
implement session creation for rHTTP preconnect.