Swapcache updates, long-term status

Matthew Dillon made some changes to swapcache(8).  Swapcache is now able to cache a lot more data, and the result is that general disk performance for _all_ disks is accelerated by an included SSD using swapcache.  Performance previously restricted to all-SSD systems or serious RAID setups is now possible with much less investment.

In addition to that, the long-term wear on the SSD appears to be less of a problem than expected.