tar: add selinux context support on create

No flag is required for the moment, it will add them
to the tar if selinux is enabled on the machine.

patchset 6: fix overflow if context is more than 512 bytes

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Change-Id: Ic7a39ee03087ed19e814b138ec6d70cdadb605cd