mission support

[abma]

add support for downloading missions so a lobby / pr-downloader can start it.

Infos from Licho, question was: why are there two types of missions on zero-k.info:
"Script only (only star script) and real big missions - mutators.

In mutators startscript is always script.txt

Mutators also always exist in plasma system too so that SL can download it if the mission is multiplayer or you want to play it using springie or something.

Im not entirely happy with this system and I think it could be improved.

One of the problems is that modinfo.lua contains dependency on the main mod and it has to be updated within mission archive every time mod changes. (Atm this happens automatically with every stable version).

Another proglem is that script is static, I think it would be better to generate it on the fly using some simpler ruleset - let server/host generate it."