Use attachment_dir only at the point where we're actually

Use attachment_dir only at the point where we're actually
reading from / writing to the files, do not carry it around
in the object. This makes us safer in the event the object
is somehow exposed to the outside world.

I may be cleaning this up some more for devel.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@12541 7612ce4b-ef26-0410-bec9-ea0150e637f0