[bugfix] [RHEL/7] Version #3 - Update form of RHEL-7 service disable / enable check macros

This change updates the form of:
```
    service-disable-macro,
    service-enable-macro,
    service-disable-check-macro,
    xinetd-service-disable-check-macro, and
    service-enable-check-macro
```
in order they to start using the form to read as ```# systemctl ...``` rather than ```# chkconfig ...```

Version 2: Replace ```<code>``` with ```<tt>```. Thanks Shawn.
Version 3: Fix s/xinet.d/xinetd.d/ typo. Thanks Gabe.

Please review.

Thanks, Jan.