com32/chain: index -1 partiter adjustment

com32/chain: index -1 partiter adjustment

Currently, the only situation when index could be set to -1 was empty
logical partition (with stepall flag active). To keep things consistent
(and with index0 already providing "full" info), adjust the partiter to
also set -1 on empty main partition.

This change also allows simple index check to see whether we're are at
the empty partition or not, without digging into disk_dos_part_entry
data and retesting ostype field. We rely on it now in manglepe_hide().

Also some retab! changes in partiter.h and one variable rename.

Signed-off-by: Michal Soltys <soltys@ziu.info>