Don't make selinux state available if selinux module isn't

Also, add notes to top-level docstring explaining that the setsebool and
semanage commands need to be available in order for the module to work,
and note which packages provide these on RHEL-based distros.

Resolves #7253.
Add additional note to selinux state docstring
Merge pull request #7260 from terminalmage/issue7253

Don't make selinux state available if selinux module isn't