Date and Time preference pane locked

Per discussion with @jvennix-r7 in pull request [2302](https://github.com/rapid7/metasploit-framework/pull/2302). 

When the date and time preference pane is locked, the exploit doesn't work. The systemsetup command takes 30 seconds to return, and the date/time preferences are not successfully set. To handle this:
1. Added a timeout to cmd_exec when it calls systemsetup.
2. Added a check after the attempt to set the date and time. If it is not 1/1/1970, fail the exploit. 

Also added a fix for a missing return in the check function.