checkout: don't apply SELinux labeling in user mode
checkout: don't apply SELinux labeling in user mode

If the user requested a user checkout, we don't want to set the SELinux
label xattr.

Closes: #903
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree