Unitsync engine support

[gajop]

Engines should be a data concept much like maps and games are, and they should be contained in their own "engines" folder (https://github.com/spring/spring/blob/4dac6507e758df6786db448db1d78c4e42b4e91a/rts/System/FileSystem/ArchiveScanner.cpp#L360-L365).

We should be able to tell which engines exist by querying VSF.GetEngineList(), and functions like Spring.Restart and Spring.Start should take an optional parameter which specifies the engine

[abma]

you should steal code from lsl:

i.e. https://github.com/springlobby/lsl/blob/master/src/lslunitsync/springbundle.cpp

[abma]

FYI: lsl was merged into pr-downloader ->

https://github.com/spring/pr-downloader/blob/master/src/lsl/lslunitsync/springbundle.cpp