bump MAXPARTITIONS to 16

Hi,

I've committed changes to bump MAXPARTITIONS from 8 to 16 as mentioned
in PR port-hpcarm/18256. (so that we can use disks created on i386)

It uses __HAVE_OLD_DISKLABEL compat method as other ports
so there should be no compat issue, but if you see any problems
please report it here or via send-pr(1).

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Izumi Tsutsui