pdftops: Write compliant ps header
According to the postscript spec, only DSC Comments are allowed in the header.
%%Creator is the header for the software used to generate the postscript file, which is pdftops in this case, and not as such the generator for the pdf file. I've chosen to, if available, keep the pdf creator as a substring in the %%Creator field.
Originates in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1068307