use selinuxenabled(8) instead of checking for /selinux

Some distros may have the SELinux pseudofilesystem mounted elsewhere,
for example /sys/fs/selinux. selinuxenabled(8) is a simpler way of
checking whether SELinux is enabled.

Bug: 1055753
Change-Id: I3afd01a7c4752b3ffb9815f6ea73d8a8c8e6b89b