Table migration improvements.  Dropped alt_image from feed table.

Table migrations in fof-install.php are now more consistent, work with all pdo drivers, and will check for missing procedures and triggers before trying to install them.  Constraints are also installed if missing. (Though migration of old MyISAM tables to a constraints-capable engine is currently left as an exercise for the user..)

Feed table no longer has alt_image column.  NOTE: No data migration occurs!