Fix setting/setting/viewing metadata that contains unicode or other special characters, when in a non-unicode locale.

Oh boy, not again. So, another place that the filesystem encoding needs to
be applied. Yay.

In passing, I changed decodeBS so if a NUL is embedded in the input, the
resulting FilePath doesn't get truncated at that NUL. This was needed to
make prop_b64_roundtrips pass, and on reviewing the callers of decodeBS, I
didn't see any where this wouldn't make sense. When a FilePath is used to
operate on the filesystem, it'll get truncated at a NUL anyway, whereas if
a String is being used for something else, it might conceivably have a NUL
in it, and we wouldn't want it to get truncated when going through
decodeBS.
(NB: There may be a speed impact from this change.)

Utility/Base64.hs

@@ -1,4 +1,7 @@
 {- Simple Base64 encoding of Strings
+ -
+ - Note that this uses the FileSystemEncoding, so it can be used on Strings
+ - that repesent filepaths containing arbitrarily encoded characters.
  -
  - Copyright 2011, 2015 Joey Hess <id@joeyh.name>
  -
@@ -9,13 +12,15 @@ module Utility.Base64 (toB64, fromB64Maybe, fromB64, prop_b64_roundtrips) where
 
 import qualified "sandi" Codec.Binary.Base64 as B64
 import Data.Maybe
+import qualified Data.ByteString.Lazy as L
 import Data.ByteString.UTF8 (fromString, toString)
+import Utility.FileSystemEncoding
 
 toB64 :: String -> String	
-toB64 = toString . B64.encode . fromString
+toB64 = toString . B64.encode . L.toStrict . encodeBS
 
 fromB64Maybe :: String -> Maybe String
-fromB64Maybe s = either (const Nothing) (Just . toString)
+fromB64Maybe s = either (const Nothing) (Just . decodeBS . L.fromStrict)
 	(B64.decode $ fromString s)
 
 fromB64 :: String -> String