Fix for a problem with the new messaging system. If a sender is using the

Fix for a problem with the new messaging system. If a sender is using the
messaging system as a notification mechanism, and the speed of notification
greatly exceeds the speed of message recovery, then you get a massively (>75Mb)
growing tdb. If the message is a simple notification, then the message is
static, and you only need one of them in transit to a target process at
any one time.
This patch adds a BOOL "allow_duplicates" to the message_send_XX primitives.
If set to False, then before sending a message the sender checks the existing
message queue for a target pid for a duplicate of this message, and doesn't
add to it if one already exists.
Also added code into msgtest.c to test this.
Jeremy.
(This used to be commit 3aa7995660395ecb85c8e35b638fa9fbbb952558)