add SectionGroup

- unify interface to platform specific section handling
- static executables have a single SectionGroup
- each shared library adds another SectionGroup
- use new DSO code for ModuleInfos
.minfo null pointers were fixed in dmd

- ELF D-Programming-Language/dmd/a1079c33960a85578fbac30c8afee7af7b0b8684
- MachO D-Programming-Language/dmd/42e7fb144e2ec10964564673c78c91b1dad012d9
Merge pull request #424 from dawgfoto/SectionGroup

add SectionGroup