Implement the getsid command

After a bit of back and forth over https://github.com/rapid7/metasploit-framework/issues/3787 I decided to go implement the `getsid` command that @Meatballs1 suggested.

This PR contains a single commit which has the following changes:

* Addition of `stdapi_sys_config_getsid`.
* Slight refactor of code that determined the user name (it was duplicated in a couple of spots). This is now in a single spot and called from a few different locations.
* Clean up of the TLV declarations (they were an unreadable mess). I'll be doing more on this later, but that can be in another PR.

In order to test this you'll need to work the MSF PR located here: