Fixed handling of zones (import/read)

- bnc#898659
- Checking for system zones was moved to separate function
- Extended list of zones marked as system (internal) ones with (0\.)+ip6.arpa
- Not allowing to edit system zones (belong to bind package)
- Added check for 'bind' package to be installed before writing the configuration
- Marking all imported zones as 'modified' to be written later
- Marking all imported non-system zones as 'is_new' to create a zone file for them
- Flushing /etc/named.conf cache 'after' writing zones (instead of 'before')
- Using "" as the default NETCONFIG_DNS_POLICY (instead of 0)