Add force option to reboot and shutdown commands

I got annoyed with my test lab telling me other users are logged on to this machine and not rebooting when I told it to!

force 1 - Shutdown hung programs
 force 2 - Try harder (and dont give the user a popup about other users).

meterpreter > reboot -h
 Usage: reboot [options]

Reboot the remote machine.

OPTIONS:
-f <opt>  Force a reboot, valid values [1|2]
-h        Help menu.

 meterpreter > reboot -f 2
 Rebooting...
 meterpreter >
 [*] 192.168.1.122 - Meterpreter session 1 closed. Reason: Died