#1 Improve "const correctness"

[Markus Elfring]

Would you like to apply the advices from the article
"http://en.wikipedia.org/wiki/Const_correctness" to
your sources?
I suggest to add the key word "const" to the type
specifiers for parameters like the following.
- fname, orientation (function "PS_Open")
font, sizemm (function "PS_set_font")

http://cvs.sourceforge.net/viewcvs.py/psplots/psplot/psplot.h?rev=3.4&view=markup

• title, fname, oriention, sizefactor (functions
  "SPS_plot", "XPS_plot")

http://cvs.sourceforge.net/viewcvs.py/psplots/psplot/xpsplot.h?rev=3.1&view=markup