fix bright bg rendering on linux term

This works very well for the term in kernel 3.0.21, but I haven't checked any other versions of vt.c.  My guess is that it is fine.

It also adds some more "which terminal am I using?" logic, which I know you try and avoid, but I don't see a way around doing this.

Without the fix in place it looks like this:
![image](https://cloud.githubusercontent.com/assets/2751728/4064557/92f9cafa-2e12-11e4-9a42-aded328b081e.png)

With the fix, it looks like this:
![image](https://cloud.githubusercontent.com/assets/2751728/4064542/7095dc2e-2e12-11e4-8c86-221dc47f2bfc.png)