Port usernode user table definitions to views_user.inc

The usernode module provides an alias of the user table, along with additional fields, filters, and sorts. I've ported these back to the main views_user.inc to give everyone the ability to display, filter, and sort by author information included in the user table. Thanks to fago for the doing the hard work. :-)

This patch adds the following fields: mail, status, account created, last access, and last login; filters: mail (using standard LIKE op), status, account created, last access, last login; sorts: uid, mail, status, created, access, login.

This patch also moves views_handler_field_email from views_profile.in to views_user.inc. This seemed more logical to me.

Also, there are two new generic field handlers: views_handler_field_value and views_handler_field_map

The first, views_handler_field_value, runs check_plain() on the $value. I know this is standard behavior if one does not specify a handler, but for email address, I wanted to specify two handlers, so I had to have something to call to create a plain text version. This handler could be used for the node title field too - instead of using the option field.

The second, views_handler_field_map allows field definitions to pass in an array of key-value pairs. For the key that matches the $value, the value from the array is returned.

Both of these handlers probably should go into views.module proper. If there are already handlers that do this, let me know and I'll reroll the patch.

On a final note: I think all the 'User: XXXX' labels should be changed to "Author: XXXX". E.g. "User: Author's Email" could be the less verbose, clearer "Author: Email". I think this makes a lot more sense about what data is being displayed, filtered, or sorted. I have not done that in this patch, but I would happily update this patch if there is support for that idea.