*: xattr can Update()

This is a gnarly patchset that has been mashed together.
It uncovered that some aspects of Check were never really working
correctly for `xattr` keywords, but also the `Update()` had been left
undone for a while.

This includes some API changes around the `Keyword` and `KeyVal` types.

Also I would like to update the signature for the `UpdateKeywordFunc` to
just accept a `KeyVal` as an argugment, rather than a keyword AND the
value. with this context there would be no need to guess on the value of
what's passed to the xattr update function of whether it needs or
already is base64 encoded.

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

check_test.go

@@ -12,7 +12,7 @@ import (
 // simple walk of current directory, and imediately check it.
 // may not be parallelizable.
 func TestCheck(t *testing.T) {
-	dh, err := Walk(".", nil, append(DefaultKeywords, "sha1"), nil)
+	dh, err := Walk(".", nil, append(DefaultKeywords, []Keyword{"sha1", "xattr"}...), nil)
 	if err != nil {
 		t.Fatal(err)
 	}