Support compression levels >= 10 and use zstd's internal default level.

zstd supports regular compression levels 1 through 22.  The special
level 0 indicates to use zstd's internal default (currently level 3).

minizip.c

@@ -573,7 +573,7 @@ int main(int argc, const char *argv[]) {
             else if ((c == 'v') || (c == 'V'))
                 options.verbose = 1;
             else if ((c >= '0') && (c <= '9')) {
-                options.compress_level = (c - '0');
+                options.compress_level = (int16_t)atoi(&argv[i][1]);
                 if (options.compress_level == 0)
                     options.compress_method = MZ_COMPRESS_METHOD_STORE;
             } else if ((c == 'b') || (c == 'B'))

mz_strm_zstd.c

@@ -313,7 +313,7 @@ int32_t mz_stream_zstd_set_prop_int64(void *stream, int32_t prop, int64_t value)
     switch (prop) {
     case MZ_STREAM_PROP_COMPRESS_LEVEL:
         if (value < 0)
-            zstd->preset = 6;
+            zstd->preset = 0; // Use zstd default.
         else
             zstd->preset = (int16_t)value;
         return MZ_OK;