Add option for skipping user xattrs.

[Matviy]

On certain file systems, such as TMPFS, user xattrs are not supported, which will cause setxattr() to fail, and current unsquashfs behavior will stop writing any xattrs from that point on.

However, it may still be desirable to write other xattrs, such as SELinux labels, which are in the security namespace, which may be supoorted (again, TMPFS).

Unpacking a live image to TMPFS for a R/W filesystem is a very common operation, and current unsquashfs code makes it impossible to preserve SELinux labels if user xattrs are also present.

This patch provides a parameter to skip the user xattrs when unpacking the squashfs.