Add hp_sys_mgmt_exec module for Linux and enhance module for Windows

The hp_sys_mgmt_exec module for Linux is a port of the Windows module with minor changes due to the requirement of quotes. It also uses Perl instead of PHP as PHP may not always be in the environment PATH. Although the Windows module works perfectly, it now uses the same technique to encode the command (thankfully, PHP adopted major syntax characteristics and functions from Perl).
Merge Linux and Windows exploit in multi platform exploit
Minor formal fixes
No autodetect. Allow the user to manually select.
Land #2181 - More targets for hp_sys_mgmt_exec

Thanks mwulftange!