[Bugfix] Port ExecShield checks to RHEL7 and Fedora

- Enable for Fedora
- Fixes #59
Slight nitpick. This should be slightly reworded, as IIRC this is a RHEL7 enforcement vs something in x64 hardware. Perhaps:

````RHEL 7, when running on x86_64 hosts, statically enables ExecShield. It cannot be enabled/disabled with sysctl.````