bug 1546 - REPLACE statements

bug 1546 - REPLACE statements

A REPLACE statement in tools/letter.pl was indirectly corrupting data
in the message_transports table.  It has been replaced with code that
does an UPDATE or INSERT depending on whether the row existed or not.

Also, to protect data in the message_transports table further, letters
which have entries in the message_transports table are no longer
deletable.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>