Fixed compiler warnings in mysqli

Fixed compiler warnings in mysqli

 - mysqli.c: Fix unused variables, they are only used in non-mysqlnd mode
 - mysqli_api.c (PHP_5_3 only): Fix constness, add_property_string expects a char *, not a const char *
 - mysqli_prop.c: Cast to long, as its below the LONG_MAX and therefore safe
 - mysqli_result_iterator.c: Cast to ulong as the iterator member expects that rather than a my_longlong

# In trunk only warnings regarding the zend_property_info
# structure is present and PHP_5_3 is warning free now