make most GC functions nothrow
add "nothrow" to ScanDG on linux and os/x, Thread.getThis in di-file

Conflicts:
	src/core/thread.di
	src/gc/gc.d
make thread_suspendAll/thread_resumeAll nothrow
mark OS/X thread function nothrow
make functions in core.sys.posix.signal and core.sys.posix.semaphore nothrow
fix build for other OSs
Merge pull request #752 from rainers/gc_nothrow3

make most GC functions nothrow