lmdb: hack to avoid windows compilation error involving mdb_filehandle_t

Relates to #33

The type is a pointer on Windows and comparing a pointer to the value -1
is non-trivial in Go.  The comparison has been replaced with a concise
but delicate hack that is untested as of yet.

lmdb/env.go

@@ -102,7 +102,7 @@ func (env *Env) FD() (uintptr, error) {
  if err != nil {
  return 0, err
  }
- if mf == C.mdb_filehandle_t(-1) {
+ if uintptr(mf) == ^uintptr(0) {
  return 0, errNotOpen
  }
  return uintptr(mf), nil