Introduce new variables FETCHINDEX, MASTER_SITE_INDEX and add description for MASTER_SITE_BACKUP.
Introduce new variables FETCHINDEX, MASTER_SITE_INDEX and add
description for MASTER_SITE_BACKUP.

Based on PR:	ports/74709
Submitted by:	Valentin Nechayev <netch@netch.kiev.ua>
Reviewed by:	ru
MFC after:	3 days

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