MINOR: check: delay MUX init when SSL ALPN is used

MINOR: check: delay MUX init when SSL ALPN is used

When instantiating a new connection for check, its MUX may be
initialized early. This was not performed though if SSL ALPN negotiation
will be used, except if check MUX is already fixed.

However, this method of initialization is problematic when QUIC MUX is
used. Indeed, this multiplexer must only be instantiated after the above
application protocol is known, which is derived from the ALPN
negotiation. If this is not the case a crash will occur in qmux_init().

In fact, a similar problem was already encountered for normal traffic.
Thus, a change was performed in connect_server() : MUX early
initialization is now always skipped if SSL ALPN negotiation is active,
even if MUX is already fixed. This patch introduces a similar change for
checks.

Without this patch, it is not possible to perform check on QUIC servers
as expected. Indeed, when http-check ruleset is active a crash would
occur prior to it.