Fix checkboxes value retrieval

+ if a checkbox is not checked, nothing sent in the GET or POST requests
+ in some places in the code, one's can read ="on" or ="1" for set value, (maybe others somewhere?)

:warning:   actually, this fix has been written while working on searchable CUSTOM FIELDS used in Advanced Search, but you are right when asking for sure: NOT extensively tested with each & every generated form

But the principle of `$checked = isset($value)` allows for ANY value used as a checked checkbox value :+1: