Illumos 5368 - ARC should cache more metadata

5368 ARC should cache more metadata
 Reviewed by: Alex Reece <alex.reece@delphix.com>
 Reviewed by: Christopher Siden <christopher.siden@delphix.com>
 Reviewed by: George Wilson <george.wilson@delphix.com>
 Reviewed by: Richard Elling <richard.elling@richardelling.com>
 Approved by: Dan McDonald <danmcd@omniti.com>

References:
  illumos/illumos-gate@3a5286a1cffceafcd8cf79c4156fad605129bf50
  https://www.illumos.org/issues/5368

Porting notes:
  1) Style change to eliminate gcc compiler warning in arc_init
     variable initialization

  2) Module parameter exported and added to the man page.

  3) Module parameter supports dynamic modification like other
     ARC tunables.

Ported by: DHE <git@dehacked.net>