&CORE::foo() for dbmopen and dbmclose

&CORE::foo() for dbmopen and dbmclose

This commit allows the subs in the CORE package for close, getc and
readline to be called through references and via ampersand syntax.  A
special case for each of them is added to pp_coreargs to deal with
calls with no arguments.  Pushing a null on to the stack (which I’m
doing for other ops) won’t work, as a null already means something for
these cases: close($f) won’t vivify a typeglob if $f is a string, so
the implicit rv2gv pushes a null on to the stack.