Per comments in the commit - setting the session cookie over and over can be troubles...

Per comments in the commit - setting the session cookie over and over can be troublesome, but doing the obvious and defaulting to *replace* cookies causes logins to fail due to something I don't care to debug - session cookie is complex through all the pages involved in a login request - instead we use a simple static cache to prevent useless duplicate cookie headers

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@14552 7612ce4b-ef26-0410-bec9-ea0150e637f0