Always generate $base_uri for every page request as opposed to doing it only on some...

Always generate $base_uri for every page request as opposed to doing it only on some pages.  Always regenerate session ID at login to prevent session fixation by an attacker who has set a malicious cookie on the client browser.  Try to clean up extraneous cookies, such as ones some browsers might actually obey from the src/ directory.  Thanks to Tomas Hoger.  (CVE-2009-1580)

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@13677 7612ce4b-ef26-0410-bec9-ea0150e637f0