Validate #required, #maxlength and #options in standard #element_validate handlers

_form_validate() does several things that could be separated into standard #element_validate handlers for greater modularity and greater flexibility for modules implementing new form element types.

The check for #required and #maxlength is done inline in _form_validate(), while e.g. #pattern is checked with a separate element_validate handler.

_form_validate() also checks that the submitted value is among the available options in the #options array. Not all elements utilizing an #options array is checked in the same way, so the function is hardcodes to give 'checkboxes' and 'tableselect' elements special treatment. Not very general, and not very friendly towards custom modules implementing new elements that use an #options array.