writefil.c: first approach at drawing diagonals for each day

This code contains the hooks to making the diagonals optional (via a
command line parameter), but that code doesn't exist yet. The lines are
drawn unconditionally at the moment.
writefil.c: second approach to drawing diagonals for each day

This approach defines a separate Postscript function to draw the
diagonal lines. The C parameter generate_diagonals is still hardcoded
to 1, so the Postscript code is always invoked.

TODO: introduce a command line argument to control whether Postscript
function draw_diagonals is called or not.
Hook up code for drawing diagonals to command-line.

It is now possible to turn on drawing of diagonals using "-i 1" on the
command-line. The default is "-i 0", which means diagonals are off.

A future extension may add the option "-i 2" for drawing diagonals from
top left to bottom right (rather than bottom left to top right).