aco: Create ways to minimize use of regex.

ACO uses regex in many situations where it is completely unneeded.  In
some cases this doubles the total processing performed by
aco_process_config.

* Create ACO_IGNORE category type for use in place of skip_category
  regex source string.
* Create additional aco_category_op values to allow specifying category
  filter using either a single plain string or a NULL terminated array
  of plain strings.
* Create ACO_PREFIX to allow matching option names to case insensitive
  prefixes.

Change-Id: I66a920dcd8e2b0301f73f968016440a985e72821