General mplayer dependency checking based on PATH

Using aptitude to check for the mplayer dependency will produce faulty results for anything except Ubuntu/Debian.

This should work for most *NIX as it relies on PATH.

Turns out the original version didn't even run on stock Ubuntu, because I used aptitude instead of apt-get for some reason. I'm not sure if this PR is the best way to go about this, though; might it be better to just run mplayer to see if it runs successfully?

(also, I'm 99% sure that remora doesn't work to begin with, and hasn't for a while)

Fixing stale code, a fun practice.