libext2fs: translate internal ext4 acl to Posix ACL in ext2fs_xattr_[sg]et()

libext2fs: translate internal ext4 acl to Posix ACL in ext2fs_xattr_[sg]et()

ext2fs_xattr_[sg]et() will now translate the Posix ACL xattrs to and
from the internal ext4 attr format, since the callers of the libext2fs
are much more likely to want to use the public Posix ACL format.

For debugfs and those applications that want to see the on-disk
format, the new ext4fs_xattr_flags() function will allow those callers
to request the raw format.

Addresses-Launchpad-Bug: #1645232

Signed-off-by: Theodore Ts'o <tytso@mit.edu>