Add undocumented inflateResetKeep() function for CAB file decoding.

The Microsoft CAB file format compresses each block with completed
deflate streams that depend on the sliding window history of the
previous block in order to decode.  inflateResetKeep() does what
inflateReset() does, except the sliding window history from the
previous inflate operation is retained.

as400/bndsrc

@@ -180,5 +180,6 @@ STRPGMEXP PGMLVL(*CURRENT) SIGNATURE('ZLIB')
   EXPORT SYMBOL("inflatePrime")
   EXPORT SYMBOL("inflateReset2")
   EXPORT SYMBOL("inflateUndermine")
+  EXPORT SYMBOL("inflateResetKeep")
 
 ENDPGMEXP

as400/zlib.inc

@@ -431,6 +431,10 @@
      D  strm                               like(z_stream)                       Expansion stream
      D  arg                          10I 0 value                                Error code
       *
+     D inflateResetKeep...
+     D                 PR            10I 0 extproc('inflateResetKeep')          End and init. stream
+     D  strm                               like(z_stream)                       Expansion stream
+      *
      D gzflags         PR            10U 0 extproc('gzflags')
       *
       /endif