[PATCH] refactor ExecCheckPermissionsModified for ACL_SELECT

Hello hackers,

Currently the code on ExecCheckOneRelPerms duplicates the logic in
ExecCheckPermissionsModified.

This change accommodates ExecCheckPermissionsModified to handle ACL_SELECT
and makes ExecCheckOneRelPerms reuse code. It also merges similar comments.

Main benefit is that it reduces LOCs and centralizes column privilege logic.

Best regards,
Steve Chavez