Merge branch 'zonefile_loading' into 'master'

Zonefile loading fixes

Most is said in the commit.
I would have refactored zone-load more (especially the custom rrset lists could be standard ptrnode_t list from list.h) and a few other things,
also it should be named something like `zonefile.h` as we already have a `zone-load.h` file which is for zone loading in general and makes it confusing.
But I didnt, as @jkadlec made a lot of changes in the rrset->rrsigs and I suspect this would result in nasty collisions (like the rrset lists code).
So... this should be done after his branch is merge-able.