[WIP] Sociomantic CDGC port

Here is initial port result available for early experiments. It can be compiled with `make -f posix.mak GC_TYPE=concurrent` and passes the test suite with only shared library tests disabled (ef20b7a).

There are still many issues to be aware of:
1) Documentation is largely missing. Working on it, reading @leandro-lucarella-sociomantic old posts (http://www.llucax.com.ar/blog/blog/tag/understanding%20the%20current%20gc) may help in the meanwhile
2) Code style differs from Phobos standards. To be fixed soon.
3) Shared library support is completely missing. Proxy infrastructure similar to one in existing gc needs to be added and I don't know if actual implementation will work in such environments or more changes will be needed.
4) Deadlock issue (http://www.dsource.org/projects/tango/ticket/2087) still remains. It is not critical to our code because it almost never uses threads so no big effort was put into it but this can be huge problem for any other project.

In general this is very far from something that can be merged upstream straight away and replace default GC on linux. It can be interesting for other projects with similar architecture and requirements and probably helpful for anyone else working on better D GC.

And contributions are welcome via pull requests towards this PR base branch (https://github.com/mihails-strasuns-sociomantic/druntime-1/tree/sociomantic-cdgc-wip) or as e-mail patches (public@dicebot.lv)
Yes, but I don't currently have version identifier to distinguish what GC is being used. This is not supposed to be merged.