Incorporate the suggested edits from the PR review.
* Rewrites helpers to just use cmd_exec, since that works in meterpreter and shell.
* Changes _EOF_ to EOF, since that threw a harmless error in shell
* Adding Joff Thyer as an @author to this commit, since I missed that in earlier
commits
* Prefer using Post mixin API instead of rolling-own implementation
* Fixes whitespace
[SeeRM #5940]