Chapter #4 - rudimentary calculator

_getop.c and _getop.h - implements getch() and ungetch() using a shared buffer
getop.c and getop.h   - implements getop() using getch() and ungetch()
stack.c and stack.h   - implements push() and pop()
calc.c and calc.h     - uses push(), pop() and getop() to implement a
                        rudimentary calculator