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/FileSystemEncoding.hs

@@ -13,6 +13,7 @@ module Utility.FileSystemEncoding (
 	withFilePath,
 	md5FilePath,
 	decodeBS,
+	encodeBS,
 	decodeW8,
 	encodeW8,
 	encodeW8NUL,
@@ -81,13 +82,21 @@ md5FilePath = MD5.Str . _encodeFilePath
 {- Decodes a ByteString into a FilePath, applying the filesystem encoding. -}
 decodeBS :: L.ByteString -> FilePath
 #ifndef mingw32_HOST_OS
-decodeBS = encodeW8 . L.unpack
+decodeBS = encodeW8NUL . L.unpack
 #else
 {- On Windows, we assume that the ByteString is utf-8, since Windows
  - only uses unicode for filenames. -}
 decodeBS = L8.toString
 #endif
 
+{- Encodes a FilePath into a ByteString, applying the filesystem encoding. -}
+encodeBS :: FilePath -> L.ByteString
+#ifndef mingw32_HOST_OS
+encodeBS = L.pack . decodeW8NUL
+#else
+encodeBS = L8.fromString
+#endif
+
 {- Converts a [Word8] to a FilePath, encoding using the filesystem encoding.
  -
  - w82c produces a String, which may contain Chars that are invalid