MFC 237646,237656,238020:

MFC 237646,237656,238020:
- Add a new line to top that provides a brief summary of the ZFS ARC memory
  usage on hosts using ZFS.  The new line displays the total amount of RAM
  used by the ARC along with the size of MFU, MRU, anonymous (in flight),
  headers, and other (miscellaneous) sub-categories.  The line is not
  displayed on systems that are not using ZFS.
- Clarify that the cached file data pages included in the "Wired" count
  in top are the BIO-level cached data (i.e. "Buf"), since the previous
  phrase was a bit ambiguous with the "Cache" count.

git-svn-id: svn://svn.freebsd.org/base/stable/8@239751 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f