Added auth.php to put authentication functions in one place. Added a

Added auth.php to put authentication functions in one place. Added a
function there to check if the user is logged in without having to
connect to the IMAP server. Using this function in compose.php as a
test.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@570 7612ce4b-ef26-0410-bec9-ea0150e637f0