cleans up numerous superfluous returns in msf/core/module

while making changes to support running a local exploit as an AutoRunScript, I noticed lib/msf/core/module.rb had numerous superfluous return statements and some unnecessarily unwieldy if statements.  There's only a few instances where the logic could be affected.  I tested those by hand to make sure I wasn't crazy.
Since the framework is so dependent on module.rb, verification should be as simple as

- [x] Travis passes
- [x] msfconsole launches and you can use it normally, load modules, run them etc.