builder, customize: disable SELinux

Having SELinux enabled (even if not enforcing) in the appliance causes
troubles to applications/libraries that read/write SELinux attributes.
customize: fix attributes of /etc/shadow (RHBZ#1146275)

When saving a configuration file, Augeas creates a new file and
replaces the old one with it; this creates a /etc/shadow file without
the SELinux xattrs, since they are missing.

Thus, create a temporary file with all the attributes of /etc/shadow, so
all the attributes of it (permissions and xattrs, among others) can be
restored properly on the new /etc/shadow.

As side effect, if a guest is already properly SELinux-labelled, then
there should be no more need to relabel it to make sure /etc/shadow
still has the right SELinux xattrs.