PHP5 compatibility and code cleanup

PHP5 compatibility and code cleanup

The first (quotefix) patch just quotes string constants when used as
indexes to arrays. This gets rid of many warning messages from PHP and
also fixes at least one case where one of the strings is a reserved word
("private") in PHP5.

The second (cleanup) patch probably needs more looking at as my lack of
understanding of PHP may mean that I've not done things properly.

1. It adds an alternative way of specifying the LDAP server, by URL
instead of hostname and port.

2. It casts the arguments of array_merge() to array types everywhere
(not sure if all of these are necessary).

3. It makes $_SESSION['ldapab']['username'] and
$_SESSION['ldapab']['binddn'] null strings if they are not set.

4. It removes the reference to the unused variable $dfexample

With these fixes, ConTagged seems to run OK on PHP 5

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