Permit processing standard input

Both of
    ./gsl/nozzle_match
    ./gsl/nozzle_match 5
work as before, but now so do
    cat ./gsl/nozzle_match | ./c99sh
    cat ./gsl/nozzle_match | ./c99sh --
    cat ./gsl/nozzle_match | ./c99sh -- -
    cat ./gsl/nozzle_match | ./c99sh -- - 5

Implementation sets filename $f and dirname $d based on arguments
and then quickly uses exec to ensure processing standard input
is the default behavior.  Some care necessary in getting 'shift'
invocations correct.

Closes #2