tasks 05*: removed priority index
portage3: new method for reading content of /etc/portage/package.*
401_users_use_flags: 2 small changes

1. dependency cleanup
2. use just added method
411_users_mask: 3 small changes

1. dependency cleanup
2. fixed filtering of exactly specified ebuilds
3. use just added method
412_users_mask: dependency cleanup
414_users_mask: use just added method
415_users_unmask: use just added method
421_users_keywords: use just added method