Fix 0012545: Make some corrections to custom field radio/checkbox output

Commit 0fea46fcda274ba9349db6091e875bb011cf11ec refactored the output of
radio and checkbox custom fields. However this refactoring introduced an
error with submitting radio fields (the name attribute should not be an
array). Additionally, currently selected values for the fields were not
being checked automatically. The check_checked() function has been
rewritten (copied from check_selected()) so they both behave the same
and can accept arrays as the first parameter.