RRSet wire: Unified RDATA-traversing function.

First draft. Requires some optimizations and testing.
Merge branch 'master' into rrset-from-wire
RRSet wire: Fixed bad argument in to_wire()
RRSet wire: rdata_from_wire_one() refactoring

Used traverse_rdata() function. Still needs some polising and
more refactoring (together with the rr_from_wire() function.
RRSet: Fixed from_wire()

Bad source size counting. Still needs polishing.
RRSet wire: Fixed const variables.
RRSet wire: Simplified traverse function.
RRSet wire: Simplified dname_to_wire() function.
RRSet wire: Fixed dname_to_wire() (max len)
RRSet wire: Moved RRSet parsing from pkt to rrset.

Also refactored a bit.
TODO: Remove the use of parse_rdata() function in utilities, so
      that it may become static.
RRSet wire: Utils not to use obsolete function.
RRSet wire: Moved to packet/
RRSet wire: Minor refactoring (MR !275).
RRSet wire: Fixed wrong assert.
RRSet wire: Bad type fixed.
rrset-wire: dname processing parameters in struct, renames and reorders
rrset-wire: consistent naming
knot_rrset_rr_from_wire: check that we are in wire
rrset from wire: safe size of destination buffer
rrset from wire: parsed data can have trailing garbage