Repair "tcp server queue overflow" brokenness, more <stdbool.h> fallout.

Return value of mbuf_len() wrong - it's returning a length value, not
a yes/no value - so when the queue length approached 65 (full!), it
still only returned "1", leading to MBUF overflow later on.  Change
from "bool" to "unsigned int", misbehaviour gone...

This fixes OpenVPN trac ticket #263.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Peter Caprioli <petercap@kth.se>
Message-Id: <20130312100706.GN953@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7396
Signed-off-by: Gert Doering <gert@greenie.muc.de>