#3 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 of parameters like the following.
- src (functions "px_encrypt_.+")

http://cvs.sourceforge.net/viewcvs.py/pxlib/pxlib/src/px_c
rypt.h?rev=1.4&view=markup

- buffer (functions "px_.*write")

http://cvs.sourceforge.net/viewcvs.py/pxlib/pxlib/src/px_i
o.h?rev=1.2&view=markup