Fall back to private db environment on system level EINVAL

Fall back to private db environment on system level EINVAL

- BDB wants to use mmap() for its environment by default, but not
  all (file)systems support this, as pointed out by Daniel Drak.
  However env->open() can return EINVAL for a number of reasons,
  require all the fallback reasons to be system level errors to
  differentiate from "logical" errors such as incompatible flags
  to (possibly pre-existing) db environment, in which case we better
  just error out.