*: update -u behavior

Fixes #16

In attempt to close https://github.com/vbatts/go-mtree/issues/16 I've
uncovered that the update was missing a function for symlink.
Additionally the update was not even opperating on the correct directory
hierarchy.

I've uncovered that os.Chtimes follows the symlink, and presumably only
Linux has an obscure way to set the mtime/atime on a symlink itself. So
I've made a custom lchtimes().

Also Mode follows through the symlink, and symlinks only ever have a
mode of 0777, so don't set them.

Lastly, directories need to have their mtime/atime set in a reverse
order after all other updates have been done. This is going to require
something like a `container/heap` to be unwound.

Also, each updateFunc will _only_ perform the update if it is needed. Much less
invasive this way.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

entry.go

@@ -112,6 +112,16 @@ func (e Entry) AllKeys() []KeyVal {
 	return e.Keywords
 }
 
+// IsDir checks the type= value for this entry on whether it is a directory
+func (e Entry) IsDir() bool {
+	for _, kv := range e.AllKeys() {
+		if kv.Keyword().Prefix() == "type" {
+			return kv.Value() == "dir"
+		}
+	}
+	return false
+}
+
 // EntryType are the formats of lines in an mtree spec file
 type EntryType int