#129 cinterf.h and const correctness

[Roberto Bagnara]

It would be nice if the interface functions declared in
cinterf.h could be declared with const correctness in
mind. For instance, the declaration

extern char* string_find(char*, int);

should be changed to (something equivalent to)

extern char* string_find(const char*, int);

[Theresa Swift]

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Finally fixed.