MINOR: mux-spop: don't add a closing connection to an idle list on detach

MINOR: mux-spop: don't add a closing connection to an idle list on detach

In spop_detach(), the test deciding whether the connection may be moved to
an idle list checks SPOP_CF_RCVD_SHUT, SPOP_CF_ERR_PENDING and
SPOP_CF_ERROR, but not the connection state. A connection that already
switched to SPOP_CS_ERROR, SPOP_CS_CLOSING or SPOP_CS_CLOSED may thus be
inserted in the session server list or in the server's idle pool, while
spop_avail_streams() reports no available stream for it.

This can only happen with a private connection, since srv_add_to_idle_list()
already rejects connections without available streams. It is not harmful
either, since session_get_conn() also requires an available stream, so such
a connection is never reused. But it is still wrong to park it: it prevents
the session from dropping it and forces a useless idle counter increment.

This is the same fix as the one applied on mux-fcgi by commit 2acf116bf
("MINOR: mux-fcgi: don't add a closed connection to an idle list on
detach"). Unlike FCGI, all states from SPOP_CS_ERROR are excluded, which
matches the test performed by spop_avail_streams().

This may be backported to 3.1.