update function not accounting for db table prefix

Issue Summary

If you are using a table prefix (eg. for multi-site db sharing) The update function doesn't work. The update attempts to modify content types in the table without apending the db-prefix to the table name. (ie. its trying to change databasename.content_table rather than the correct databasename.PREFIX_content_table.)

Ive never written a module, but in looking at some other database upgrade scripts, people are using more abstract functions like "content_alter_db_field" rather than editing the database directly.