Rename addmessageheader()'s crlf parameter to linebreak

The parameter's value is a string representing the linebreak,
and can sometimes contain just '\n', in which case naming it
crlf is slightly misleading.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
addmessageheader: fix case #2 and flesh out docstring

The example illustrations were slightly cryptic;
modify them to be more obvious.

Fix case #2 (message starts with two line breaks)
where additional line break was shown and coded.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
addmessageheader(): add debug for header insertion

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>