[gc] add -D2xx MEM_DETAIL_DEBUG_FLAG

pass -D for parrot also down to gc_initialize, not only parrot_old
undo --gc-dynamic-threshold=2 for gms, use the standard --gc-nursery-size
as documented.
add seperate GC_DEBUG_DETAIL_2 tracer to seperate it from
MEM_DETAIL_DEBUG_FLAG tracing for every single alloc/free.
[pmc] add Coroutine.mark with caller_seg

Coroutine.mark was just calling Sub.mark and missed to mark
the caller_seg. Do it now properly with inheritance.
Note that this does not fix GH #1109, which is a Coro in a wrong gen