Prevent xattr remove from creating xattr directory

Attempting to remove an xattr from a file which does not contain
any directory based xattrs would result in the xattr directory
being created.  This behavior is non-optimal because it results
in write operations to the pool in addition to the expected error
being returned.

To prevent this the CREATE_XATTR_DIR flag is only passed in
zpl_xattr_set_dir() when setting a non-NULL xattr value.  In
addition, zpl_xattr_set() is updated similarly such that it will
return immediately if passed an xattr name which doesn't exist
and a NULL value.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Improve xattr property documentation

Extend the xattr property section of zfs(8) such that it covers
both styles of supported xattr.  A short discussion of the benefits
and drawbacks of each type is presented to allow users to make an
informed choice.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Posix ACL Support

This change adds support for Posix ACLs by storing them as an xattr
which is common practice for many Linux file systems.  Since the
Posix ACL is stored as an xattr it will not overwrite any existing
ZFS/NFSv4 ACLs which may have been set.  The Posix ACL will also
be non-functional on other platforms although it may be visible
as an xattr if that platform understands SA based xattrs.

By default Posix ACLs are disabled but they may be enabled with
the new 'aclmode=noacl|posixacl' property.  Set the property to
'posixacl' to enable them.  If ZFS/NFSv4 ACL support is ever added
an appropriate acltype will be added.

This change passes the POSIX Test Suite cleanly with the exception
of xacl/00.t test 45 which is incorrect for Linux (Ext4 fails too).

  http://www.tuxera.com/community/posix-test-suite/

Signed-off-by: Massimo Maggi <me@massimo-maggi.eu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #170