Silence unused variable and parameter warnings

Silence unused variable and parameter warnings

- Cast unused variables and parameters to void, do nothing
- Fixes the following compiler warnings on Windows:
rrd_open.c:
  757:9: warning: unused parameter 'writelock' [-Wunused-parameter]
    int writelock)
  810:24: warning: unused variable 'rrd_simple_file'
    [-Wunused-variable] rrd_simple_file_t *rrd_simple_file;
  807:17: warning: unused parameter 'rrd_file' [-Wunused-parameter]
    rrd_file_t *rrd_file,
  808:12: warning: unused parameter 'rrd' [-Wunused-parameter]
    rrd_t *rrd)
rrd_client.c:
  746:39: warning: unused parameter 'client' [-Wunused-parameter]
    static int connect_unix(rrd_client_t *client, const char *path)
    /* {{{ */
  746:59: warning: unused parameter 'path' [-Wunused-parameter]
    static int connect_unix(rrd_client_t *client, const char *path)
    /* {{{ */