Field attribute UNSIGNED

Depending on the unsigned property value:

- TRUE will always set $field['unsigned'] to 'UNSIGNED'
- FALSE will always set $field['unsigned'] to ''
- array(TYPE) will set $field['unsigned'] to 'UNSIGNED',
if $attributes['TYPE'] is found in the array
- array(TYPE => UTYPE) will change $field['type'],
from TYPE to UTYPE in case of a match

@param array &$attributes
@param array &$field

@return null|void