more cookie security FS#1490

This patch adds the httponly option to the PHP session cookies and DokuWiki's
auth cookie when supported by the PHP version.

It also adds a new config option 'securecookie' which is enabled by default.
It makes sure the browser will not sent a cookie set via HTTPS over a
non-secured connection. This option has to be disabled for wikis that only
protect the login with SSL but not the whole wiki.

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