iprt/cdefs.h: Refactored the typedef use of DECLCALLBACK as well as DECLCALLBACKMEMBER to wrap the whole expression, similar to the DECLR?CALLBACKMEMBER macros. This allows adding a throw() at the end when compiling with the VC++ compiler to indicate that the callbacks won't throw anything, so we can stop supressing the C5039 warning about passing functions that can potential throw C++ exceptions to extern C code that can't necessarily cope with such (unwind,++). Introduced a few _EX variations that allows specifying different/no calling convention too, as that's handy when dynamically resolving host APIs. Fixed numerous places missing DECLCALLBACK and such. Left two angry @todos regarding use of CreateThread. bugref:9794