As inauguration, move G_AddGroup, G_AddPath and struct strllist there.
The header is located in build/include, because in the future, code that resides
closer to (but is not strictly part of) the engine might need to be factored
into here. The source file, however, is in the source/ directory.
git-svn-id: https://svn.eduke32.com/eduke32@2542 1a8010ca-5511-0410-912e-c29ae57300e0
