More parser work: fixed a few bugs with [[intlinks]] in headers, a bug that caused the paragraph parser to return an empty string, and added a warning/backup-and-restore for when a render stage returns an empty string.

# Used for testing mod_rewrite.
<IfModule mod_rewrite.c>
RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule (.*) install.php?do=modrewrite_test&str=$1
</IfModule>