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>
(cherry picked from commit 0eb398501fab9c016b9b6008682c43873c4a6188)