Shared library in D on Linux

[Martin Nowak]

> With gdc I get different errors, but it seems even more difficult to get  
> it working.
>
> Does anyone know what is missing to get proper shared library support  
> working on Linux?
>

Most prerequisites are merged now but there are still some smaller things  
I need to do.

https://github.com/dawgfoto/dmd/tree/SharedRuntime
  - split off EH changes (github.com/D-Programming-Language/dmd/pull/821)

https://github.com/dawgfoto/druntime/tree/SharedRuntime
  - split off EH changes  
(github.com/D-Programming-Language/druntime/pull/185)
  - TLS ranges must not be updated in SIGUSR1
  - integrate dylib_fixes.c (weak main, automatic rt_init/rt_term)
  - integrate shared library tests
  - cleaner makefile and unittests

https://github.com/dawgfoto/phobos/tree/SharedRuntime
  - cleaner makefile and unittests

This should be all to support link-time shared libraries.

For runtime loading we'd need:

  - a GC hook to mark/finalize auxiliary memory
  - traversal of DSO dependency DAGs for thread
    local initialization