Adapt filesystem-independent NFSv4 ACL code (used by UFS, but not by ZFS)

Adapt filesystem-independent NFSv4 ACL code (used by UFS, but not by ZFS)
to PSARC/2010/029.  In short, the semantics is simplified - "weird stuff"
no longer happens after chmod, entries don't get duplicated during
inheritance, and trivial ACLs no longer contain three "DENY" entries,
which is also more friendly to MS Windows.

By default, UFS keeps using old semantics.  To change it, set sysctl
vfs.acl_nfs4_old_semantics to 0.  I'll flip the switch when ZFSv28
hits the tree, to keep these two in sync - ZFS v28 uses PSARC semantics,
and ZFS v15 uses the old one.