File ffcall/callback/vacall_r/README from the latest check-in

This directory contains a reentrant version of the vacall package.

Instead of using a global variable for the function to be called,
a pointer is passed in a special CPU register, pointing to:
  - a function code pointer in the first word,
  - a data word in the second word, to be passed before the va_alist.

The include file <vacall.h> is renamed to <vacall_r.h>.