pbrd: use flags to indicate active fields

pbrd: use flags to indicate active fields

    Before now, PBRD used non-zero values to imply that a rule's
    match or action field was active. This approach was getting
    cumbersome for fields where 0 is a valid active value and
    various field-specific magic values had to be used.

    This commit changes PBRD to use a flag bit per field to
    indicate that the field is active.

Signed-off-by: G. Paul Ziemba <paulz@labn.net>