Set the operationHandlers[op].DecodeIn function and
add xattr name parsing.

SETXATTR is special because it is the only opcode that
takes a file name (the xattr name) and a binary blob
(the xattr value). This was not supported by the
file name parsing code:
* setting FileNames = 1 would lump the xattr name
  and value together and truncates the last byte
* setting FileNames = 2 truncated the last xattr
  value byte.

This was solved by adding a special-case to parse(),
which seemed less ugly than adding a special-case
InputDebug(), or leaving the truncation as-is.

Before:
  2018/05/01 16:47:39 Dispatch 6: SETXATTR, NodeId: 3.  12 bytes
After:
  2018/05/01 16:48:36 Dispatch 6: SETXATTR, NodeId: 3. data: {sz 3 f0} names: [user.foo] 12 bytes

The change only affects debug output as doSetXAttr() does its
own bytes.SplitN(). The parsed filename *could* also be used in
doSetXAttr(), but the code seems clearer as-is.