potx 6.x-1.2

Fixed several bugs and got the 5.x and 6.x parser in line:

- #205100 by phirdina: plural translations were missing in exported po files due to bugos database logic
- Forward porting the Unknown file version handler from 5.x

Getting potx-6.1 and 6.2 in sync:
   - updated documentation to match actual behaviour (folding, tab title)
   - removed hook_node_types() support, as it is a Drual 4.6 construct, not supported anymore
   - ported back hook_perm() error detection from 6.2

- #213801 report by nancyw, patch by myself: replace old permission name with new one
- Mark runtime strings as duplicates as well, when we encounter them
- #205360 by damz with additions from myself: extract region names, include default region names; add default user role names

Upgrading is highly recommended.