Warn when sending very large messages

Warn when sending very large messages

Depending on configuration, messages of a certain size sent through a
Jabber server will cause the jabber server to disconnect the client.
This change allows admins to configure a message size warning threshold.
When a message meets or exceeds the size threshold, a warning is issued
to the logs with the message size (in bytes) and the message recipient.
It does not prevent the message from being delivered.  It's purely
informational.

Use 1 800 000 as the default threhold.

Signed-off-by: Bill Erickson <berick@esilibrary.com>