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.
* Added some code comments and stuff.

In order to test this you'll need to work the MSF PR located here: https://github.com/rapid7/metasploit-framework/pull/4150