Cache the base url (protocol://host:port) in the session to make it

Cache the base url (protocol://host:port) in the session to make it
faster. get_location already parsed php_self to get the url
e.g. /path/to/sqmail/src
So now, to make Locations compliant, you can either, call get_location if
you know the place you're redirecting to has the same url path you do
(src to src, for example), or check for 'sq_base_url' in the session vars,
call get_location to set it if it isn't there, and then use the
'sq_base_url' session var + base_uri from session + desired location to
redirect.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@4629 7612ce4b-ef26-0410-bec9-ea0150e637f0