VM idle page zeroing

[Matthew Dillon]

    Venkatesh's VM idle page zeroing code has been committed.  I used a
    squashed merge and rolled in some additional fixes for problems that
    came up during testing, so our main repo probably won't sync with
    Venkatesh's repo now.

    It currently defaults to disabled.  We will change the default to
    enabled once we do a little more testing.  'sysctl vm | fgrep zero'
    gets you all the sysctls.

    It doesn't make a huge difference when testing a parallel buildworld,
    *maybe* saving 20 seconds on a 1200 second build.  I expect that it
    will make more of a difference for burst or serialized operations such
    as shell script execution where cpus have enough idle time to
    pre-zero more pages.

    The page zeroing rate has been bumped up to levels more condusive to
    expected workloads, around 20,000 pages per second max.

						-Matt