add PERLSI_MULTICALL

add PERLSI_MULTICALL

Currently when MULTICALL pushes a new stackinfo, it uses the si_type
PERLSI_SORT. This is probably just a hangover from when the MULTICALL code
was created from similar code used to implement sort.

Change it to use the new PERLSI_MULTICALL si_type.

The si_type value is mainly used for debugging/dumping purposes,
apart from a few places where we check for whether this is
the top stack (PERLSI_MAIN); or check for a sort BLOCK (cxix = 0,
CXt_NULL, PERLSI_SORT).

So this commit should have no immediate effect. It will in future
however allow us to detect whether we have a sort or a true MULTICALL.