ustack helper and MDB: fixes #6309 and several related issues

This change makes several improvements to the ustack helper and MDB
support:

- ustack helper and MDB: add support for two-byte strings
  (necessary to print many filenames in stacktraces in 0.10 and later).
- ustack helper: fix position numbers, which were off by a factor of two
- ustack helper: fix frames with undefined Scripts (e.g., "RegExp")
- ustack helper: add stub frames
- MDB: add support for sliced strings
- MDB: sync up with changes from the illumos version of the module