Add trivial acl helpers

Add trivial acl helpers

The zfs acl code makes use of the two OpenSolaris helper functions
acl_trivial_access_masks() and ace_trivial_common().  Since they are
only called from zfs_acl.c I've brought them over from OpenSolaris
and added them as static function to this file.  This way I don't
need to reimplement this functionality from scratch in the SPL.

Long term once I take a more careful look at the acl implementation
it may be the case that these functions really aren't needed.  If
that turns out to be the case they can then be removed.