[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.

lib/decompress/zstd_decompress_internal.h

@@ -123,6 +123,9 @@ struct ZSTD_DCtx_s
     const ZSTD_DDict* ddict;     /* set by ZSTD_initDStream_usingDDict(), or ZSTD_DCtx_refDDict() */
     U32 dictID;
     int ddictIsCold;             /* if == 1 : dictionary is "new" for working context, and presumed "cold" (not in cpu cache) */
+    int dictUsesRemaining;       /* if == 1 : dictionary should be used once.
+                                  * if == 0 : dictionary should be forgotten now.
+                                  * if < 0 : dictionary should be used indefinitely. */
 
     /* streaming */
     ZSTD_dStreamStage streamStage;