Changed the DIY-nerds system to Basic Small Systems.

This is more in line with what was being done in 0.5.3: it's not
worth very much, but it's *something*. I don't think having a 40%
chance of a mission literally giving you absolutely nothing is a
good idea.
Added a description to Steiner.
Changed planet classes from enum to plain strings.

The Naev engine wasn't doing anything in particular with explicitly
defined classes. If anything were to be done, it would be in Lua,
and that only sees strings anyway. Just changing it to strings makes
it more flexible without losing anything.
Merge branch 'uhrabi' of https://github.com/onpon4/naev into tmp
Merge branch 'curgen_nerds' of https://github.com/onpon4/naev