plugin/file: Rename do to walk and cleanup and document (#2987)

* plugin/file: Rename do to walk, cleanup and document

* This renames Do to Walk to be more inline with Go standards. Also make
  it return an error instead of a bool.
  Also give give walk access to rrs. Alternatively e.m could be
  exported, but just access the map of rrs should work as well. Another
  alternative would be adding a whole bunch of helper functions, but
  those need grab and return the data. Just having access to the rrs
  should be easiest for most Walks.
* It adds Type and TypeForWildcard to show the different functions
* *Removes* the identical RR check when inserting; this was only done
  for A, AAAA and MX and not finished; removed under the mantra garbage
  in garbage out.
* Reuses Types to return all the types in an *tree.Elem

Signed-off-by: Miek Gieben <miek@miek.nl>

* better comments

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/file.go

@@ -119,7 +119,6 @@ func (s *serialErr) Error() string {
 // If serial >= 0 it will reload the zone, if the SOA hasn't changed
 // it returns an error indicating nothing was read.
 func Parse(f io.Reader, origin, fileName string, serial int64) (*Zone, error) {
-
 	zp := dns.NewZoneParser(f, dns.Fqdn(origin), fileName)
 	zp.SetIncludeAllowed(true)
 	z := NewZone(origin, fileName)