Fix incorrect uses of gzFile

gzFile is already a pointer, so code like

gzFile *handle = gzopen(...)

is wrong.

This used to pass silently because gzFile used to be defined as void*,
and you can assign a void* to a void**.  But somewhere between zlib
versions 1.2.3.4 and 1.2.6, the definition of gzFile was changed to
struct gzFile_s *, and with that new definition this usage causes
compiler warnings.

So remove all those extra pointer decorations.

There is a related issue in pg_backup_archiver.h, where

FILE       *FH;             /* General purpose file handle */

is used throughout pg_dump as sometimes a real FILE* and sometimes a
gzFile handle, which also causes warnings now.  This is not yet fixed
here, because it might need more code restructuring.

src/bin/pg_dump/pg_backup_tar.c

@@ -58,16 +58,13 @@ static void _EndBlobs(ArchiveHandle *AH, TocEntry *te);
 #define K_STD_BUF_SIZE 1024
 
 
-#ifdef HAVE_LIBZ
- /* typedef gzFile	 ThingFile; */
-typedef FILE ThingFile;
-#else
-typedef FILE ThingFile;
-#endif
-
 typedef struct
 {
-	ThingFile  *zFH;
+#ifdef HAVE_LIBZ
+	gzFile		zFH;
+#else
+	FILE	   *zFH;
+#endif
 	FILE	   *nFH;
 	FILE	   *tarFH;
 	FILE	   *tmpFH;