Implement File Attribute Support

We add support for lsattr and chattr. Only attributes common to both
Solaris and Linux are supported. These are 'a', 'd' and 'i'. File
attributes exclusive to Solaris are present in the ZFS code, but cannot
be accessed or modified through this method. That was the case prior to
this patch.

This commit removes the ZFS_IOC_GETFLAGS and ZFS_IOC_SETFLAGS macros in
recognition that this is not equivalent to the Solaris operation. The
resolution of issue zfsonlinux/zfs#229 should implement something
equivalent that will permit access and modification of Solaris-specific
attributes.

This resolves a regression caused by
88c2839 that broke python's
xattr.list(). This broke Gentoo Portage's FEATURES=xattr, which depended
on this.

https://bugs.gentoo.org/show_bug.cgi?id=483516
Issue zfsonlinux/zfs#1691

Original-patch-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>