Rename type OpenFlags to OpenResponseFlags

All the other OpFlags (InitFlags, ReleaseFlags, WriteFlags) are flags
on the request, but OpenFlags was flags on the response, and
OpenRequest has a Flags field of its own. This should minimize
confusion down the line.
Do not repeat operation and header in debug output "<-" lines
Remove ReadRequest.Mode, it was never filled

The only reason it was there was because of false sharing of openIn
struct with opCreate. File/directory create requests contain a mode,
but Open is always referring to a pre-existing entry.
Test chmod
Test OpenRequest.Flags; zero hard-to-handle O_LARGEFILE bit

The os.O_LARGEFILE flag Go exposes is not the O_LARGEFILE Linux has,
and more importantly Go's version has value 0 for amd64. This is
annoying to handle, and what ABI/API the client side chooses to talk
to VFS is irrelevant for the protocol talked VFS->FUSE->us.
Test CreateRequest.Flags, Mode; zero hard-to-handle O_LARGEFILE bit
Refactor create flag mangling into openFlags
Fix and test MkdirRequest.Mode

Force mode to be directory in mkdir before calling fileMode, not after
it. On Linux 3.11, in.Mode == 0755, specifically in.Mode&S_IFMT==0,
and this caused fileMode() to go into its "no idea" branch and mark it
as os.ModeDevice. The resulting mode was `dDrwxrwxr-x`.
ReleaseRequest.Flags is an O_APPEND-style flag too
Give OpenFlags its own type, pretty-print

This adds some type safety, but most importantly makes the debug log
easier to read.
Make git ignore common editor autosave and backup files

Thanks to David Roundy.