zlib: fix write request reference counting

Keep track of the reference count, don't make the wrapper object weak
when there are pending write requests.  Fixes a regression from c79d516.
dtrace, mdb_v8: support more string, frame types

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

Fixes #6309
Closes #6318
mdb_v8: remove useless check