fuse: Attr() now has a Context parameter and error return value

~GOPATH/src/bazil.org/fuse:master$ git shortlog 48c34fb7780b88aca1696bf865508f6703aa47f1..e4fcc9a2c7567d1c42861deebeb483315d222262
Tommi Virtanen (8):
      Remove dead code
      Make saveLookup take Context, return error
      Make serveNode.attr take Context, return error
      Make nodeAttr take Context, return error
      API change: Move attribute validity time inside Attr
      Set attribute validity default time in one place
      API change: Attr method takes Context, returns error
      Set LookupResponse validity times up front, instead of after the handler