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)
It looks like it has fixed the problem, but frankly, it might be still caused by some other inconsistency or misconfiguration.

Running system doesn't use the default as you have to set some value for this variable. Running system doesn't break the resolv.conf. Anyway, I still need to test this properly on a clean system.