Store common Linux xattrs as native SA with xattr=sa

This patch adds a new class of system atributes which are referred to as
"Native SA xattrs".  The facilities described here are only enabled when
"xattr=sa" is set.

If xattr=sa is set, the following "security." and "system." xattrs are
stored as native SA xattrs rather than as elements of the ZPL_DXATTR SA:

          xattr                      System atrribute
    --------------------------------------------------------
    security.selinux            ZPL_SECURITY_SELINUX
    security.capability         ZPL_SECURITY_CAPABILITY
    system.posix_acl_access     ZPL_SYSTEM_POSIX_ACL_ACCESS
    system.posix_acl_default    ZPL_SYSTEM_POSIX_ACL_DEFAULT

Storing these xattrs as native system attributes allows for the ZPL to
more easily and naturally operate on them as an atomic part of other
operations and will be used as the foundation for fixing issue #2718.

Zdb will display these under the new "Native SA xattrs" section.

Lookups of these xattrs will use the following priority:

    1. Native SA xattr (as shown in the able above)
    2. Linux ZPL_DXATTR nvlist
    3. Traditional ZFS directory-style xattr

Modifications of these xattrs will erase an existing ZPL_DXATTR instance
of an identically-named xattrs but will not change an existing instance
of an identically-named directory-style xattr.

If a modification of the ZPL_DXATTR SA causes it to become empty, the
ZPL_DXATTR SA, itself is deleted to maximize available space in the
bonus buffer.

The effect is that existing ZPL_DXATTR SAs are automatically upgraded
as they are changed.