Users without capability "create_users" can add new users

[imwebgefunden]

If a user has the capability "edit_users" and not the capability "create_users" he can add new users.
The defect is in admin-ajax.php. The check is against "edit_users" and not "create_users". I've attached a patch to fix this issue.
A second one - more an AddOn and not an defect: We should show the add user form only if the current user has the capability to add a new user. If the current user has the capability "create_users" the form will be shown. The second patch I attached make this job.

Patch for admin-ajax.php to check against "create_users"

Show "Add User" Form only if the current user can create new users

(In [7659]) Check create_users cap instead of edit_users wgen adding/inserting users. Props imwebgefunden. fixes #6662 for 2.5

(In [7660]) Check create_users cap instead of edit_users wgen adding/inserting users. Props imwebgefunden. fixes #6662 for trunk