#2 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.
- str (functions "parse_conversion", "hex2char")

http://cvs.sourceforge.net/viewcvs.py/dcfldd/dcfldd/dcfldd.h?rev=1.7&view=markup

- pattern (functions "make_pattern", "replicate_pattern")

http://cvs.sourceforge.net/viewcvs.py/dcfldd/dcfldd/pattern.h?rev=1.3&view=markup

[Markus Elfring]

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How do you think about the suggested API polish?