new remediation for CCE-27272-4 (tftp secure mode)

imported from stig-fix project.
Couple of issues with this proposal:
* the script instructs ```tftpd``` to change root directory on startup to be a non-existing ```/tftpboot``` one (effectively meaning IMHO tftpd won't change root directory on startup. It should be using ```/var/lib/tftpboot``` directory instead which is ensured to exists, when ```tftp-server``` package is installed since it comes with that package:
```
# rpm -ql tftp-server
/etc/xinetd.d/tftp
/usr/sbin/in.tftpd
/usr/share/doc/tftp-server-0.49
/usr/share/doc/tftp-server-0.49/CHANGES
/usr/share/doc/tftp-server-0.49/README
/usr/share/doc/tftp-server-0.49/README.security
/usr/share/doc/tftp-server-0.49/README.security.tftpboot
/usr/share/man/man8/in.tftpd.8.gz
/usr/share/man/man8/tftpd.8.gz
/var/lib/tftpboot
```
* for the work of the remediation itself - when ```server_args``` row is present in ```/etc/xinetd.d/tftp``` it will effectively remove all existing server_args options, and use just ```server_args = -s /tftpboot```. Meaning if the administrator (pre)-configured ```server_args``` to read e.g. ```-c -v``` these would be erased, and just ```-s /tftboot``` would be used. IMHO if the -s option isn't present in server_args options, we should just append it together with path, but not reset the existing configuration.
* the script doesn't handle properly the case when ```server_args``` is completely missing from ```/etc/xinetd.d/tftp``` file (it's commented our or completely missing). In this case the remediation fails (nothing is performed). In both cases (commented out or not present in the file at all) the proper setting should be appended. =>

We have two possibilities here (where either way works for me):
* merge the current proposal & enhance it (to properly deal with issues above) or,
* update this version to reflect them.

To summary I am not against merging this under assumption we fix those issues above.