Allow ampersand calls for CORE subs with $*$$**$ protos

Allow ampersand calls for CORE subs with $*$$**$ protos

This enables ampersand calls and calls through references for CORE
subs that have * and $ in their prototypes and a fixed number of
arguments.

Usually, the *-prototyped ops have their child ops wrapped in rv2gv’s
(*{}) implicitly.  The rv2gv op is sometimes flagged as an autoviv-
ificatory op, such as the first argument to accept() or open().
S_is_handle_constructor contains the list of ops that turn on
that flag.

This commit makes the coreargs op use a couple of flags to serve the
same purpose.  pp_coreargs itself calls S_rv2gv (split out from
pp_rv2gv recently for precisely this purpose) with arguments based on
its own flags.

Currently the autovivified glob gets a name like main::_GEN_0 instead
of main::$a.  I think we can live with that.