Bug#688814: pdfunite: error-prone command line arguments

[Peter Kozich (UM)]

3 more years have passed and pdfunite still keeps harassing its users.
($ /bin/pdfunite --version
pdfunite version 0.62.0
Copyright 2005-2017 The Poppler Developers - 
http://poppler.freedesktop.org)

Both earlier suggestions (-o switch and error if output file exists)
are fine.

Just to rewake the motivation I add a 3rd one: print the output to
stdout and it's up to the user to redirect it to whereever they wish.
This could coexist with the -o switch.

PK