Enable xattr tests

Enable xattr tests

Updated the xattr_common.ksh helper functions to use the attr
command on Linux to manipulate xattrs.  Added an xattr.cfg file
and reworked the user/group functionality to be consist with
the existing delegate test cases.  The intent of each test
case was preserved.

* xattr_001_pos, xattr_002_neg - Updated to verity xattr=on
  and xattr=sa sytle xattrs.

* xattr_003_neg - Use user_run helper instead of su.

* xattr_004_pos - Updated to work with ext2 xattrs.

* xattr_007_neg - Updated to use attr instead of runat.

* xattr_008_pos, xattr_009_neg8_pos, xattr_010_neg -
  Test cases disables since they aren't applicable to Linux.

* xattr_011_pos - Updated to expected behavior from GNU
  versions of the tested utilities.

* xattr_012_pos - Updated to use xattrtest to create many
  small xattrs instead of a single large one.

* xattr_013_pos - Updated to use attr instead of runat.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6128