&CORE::foo() for (sys)read and recv

&CORE::foo() for (sys)read and recv

These are grouped together because they all have \$ in their
prototypes.

This commit allows the subs in the CORE package under those names to
be called through references and via &ampersand syntax.

The coreargs op in the subroutine is marked with the OPpSCALARMOD
flag.  (scalar_mod_type in op.c returns true for these three ops,
indicating that the OA_SCALARREF parameter is \$, not \[$@%(&)*].)

pp_coreargs uses that flag to decide what arguments to reject.