- Added argument to berrno::strerror() to pass errno.

- Cleaned up a few more old strerror() calls and eliminated a
  few of the now unnecessary set_errno() calls.
- Fixed a bug in the polling code that prevented more than 100
  label reads (insanity check disabled if polling).
- Ignore bad return from open_device() if polling.
- Short circuit code if fatal error return from read_label() in
  mount.c