support for checkbox groups  - multiple params with same names

A set of xxx_param_group functions for this case:

```
<input type="checkbox" name="chk_group" value="value1" />Value 1
<input type="checkbox" name="chk_group" value="value2" />Value 2
<input type="checkbox" name="chk_group" value="value3" />Value 3
```

Functions return list of values of all request get/post/get++post params with given name. Default return value for "no parameters found" case is [], unlike xxx_param functions, where default return value is 'undefined'.

New functions, so no backward incompatibility.