sort(!) out CXt_NULL and CXp_MULTICALL

sort(!) out CXt_NULL and CXp_MULTICALL

A sort BLOCK is done using a CXt_NULL context type. Currently it has
the CXp_MULTICALL flag set. Remove this flag so that CXp_MULTICALL is
only set on CXt_SUB contexts.

Also add code comments in various places explainging that CXt_NULL is
likely to a sort BLOCK, and fix the comments in pp_return which said
a particular code path was only taken by sort BLOCK; it's also taken
be (?{...}) too.