Separate menu callback when deleting comment(s), forum and user(s)

Based on the comments in http://drupal.org/node/84076 and http://drupal.org/node/83007, the workflow of deleting forum and multiple comments can be improved. The workflow of deleting multiple users can also be improved.

This patch does the following for deleting multiple comments, multiple users, and forum/container:

1. Designate separate menu callback for deletion of forums, comments, and users respectively.
2. Operation will be checked at the _submit hook of the admin overview page of each module. The respective delete menu callback will be called accordingly.
3. Usage of $_POST along the process was removed.

For comment and user module only:
1. Since $_POST is not used anymore, the array of multiple comments and users cannot be passed to the callback. To solve this, cid of comments and uid of users are appended respectively at the end of the delete menu item, imploded with the '+' symbol.
2. The string of cids/uids are exploded into arrays at the callback function. The deletion process after that remains the same.

The confirm form for deleting forum/container was also changed to reflect the type of term to be deleted.

Please review, maybe the patch could be improved further or maybe breaking it up is better?