Assure that gzoffset() is correct when appending.

An open() with O_APPEND followed by an lseek() to determine the
position will return zero for a non-empty file, even though the
next write will start at the end of the file.  This commit works
around that by doing an lseek() to the end when appending.

gzlib.c

@@ -248,8 +248,10 @@ local gzFile gz_open(path, fd, mode)
         free(state);
         return NULL;
     }
-    if (state->mode == GZ_APPEND)
+    if (state->mode == GZ_APPEND) {
+        LSEEK(state->fd, 0, SEEK_END);  /* so gzoffset() is correct */
         state->mode = GZ_WRITE;         /* simplify later checks */
+    }
 
     /* save the current position for rewinding (only if reading) */
     if (state->mode == GZ_READ) {