SL-10190: Slightly reduce conditional clutter in llcoros.{h,cpp}.

Rename 'winlevel()' to 'sehandle()'; change it from a static member function
to a free function, thus eliminating the conditional in llcoros.h.

Elsewhere than Windows, provide a zero-cost pass-through sehandle()
implementation, eliminating the conditional in toplevel().

# Conflicts:
#	indra/llcommon/llcoros.cpp
#	indra/llcommon/llcoros.h