koma letter exporter: changing the priority of options

[Alan Schmitt <alan.schmitt@polytechnique.org>]

Hello,

I just had to write a new letter with a fresh LCO file, and I would like
to propose to change the priority of options. The current priority is:
local options > emacs variables > lco file.

Unfortunately emacs variables have a default value, which means they are
output in the .tex file even if they are not set. Thus it is impossible
to set some options in the lco file (such as foldmarks or backaddress).

I propose to either change the priority to:
local options > lco file > emacs variables

or to not output these options when they have not been set. Here are the
four options I have not set that end up in my .tex file, shadowing my
lco configuration:

,----
| \KOMAoption{backaddress}{true}
| \KOMAoption{foldmarks}{true}
| \KOMAoption{fromphone}{true}
| \KOMAoption{fromemail}{true}
`----

What do you think?

Alan