Read shortcut, buffer cache improvements

The new vm.read_shortcut option has been turned on by default by Matthew Dillon, which should lead to some performance improvements.  That improvement has been measured for tmpfs, at least.  There’s also some buffer cache improvments that help on x86_64 systems, too.

Update: As Venkatesh Srinivas pointed out, tmpfs also no longer uses the mplock, so it’ll take better advantage of multiple processors.