Changes since last update:
- Fix firstfsb variables that we left uninitialized, which could lead to
  locking problems.
- Check for NULL metadata buffer pointers before using them.
- Don't allow btree cursor manipulation if the btree block is corrupt.
  Better to just shut down.
- Fix infinite loop problems in quotacheck.
- Fix buffer overrun when validating directory blocks.
- Fix deadlock problem in bunmapi.