the pathname of the swap device is saved into. add a char *swd_path
member to struct swapdev, that contains a copy of the pathname
(using malloc(9)). rename swapctl(2)'s SWAP_STATS to SWAP_OSTATS,
and add a new SWAP_STATS command (number). make swapctl(SWAP_STATS,
...) [new version] copy the path out. if COMPAT_13, also include
support for SWAP_OSTATS. also fix a minor bug in swapctl(2).
the point of this is that swapfiles are now shown in `swapctl -l'.
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