[libzstd] Fix decompression dictionary bugs and clean up initialization

[libzstd] Fix decompression dictionary bugs and clean up initialization

Bugs:

* `ZSTD_DCtx_refPrefix()` didn't clear the dictionary after the first
  use. Fix and add a test case.
* `ZSTD_DCtx_reset()` always cleared the dictionary. Fix and add a test
  case.
* After calling `ZSTD_resetDStream()` you could no longer load a
  dictionary, since the stage was set to `zdss_loadHeader`. Fix and add
  a test case.

Cleanup:

* Make `ZSTD_initDStream*()` and `ZSTD_resetDStream()` wrap the new
 advanced API, and add test cases.
* Document the equivalent of these functions in the advanced API and
  document the unstable functions as deprecated.