Improve const-correctness

[elfring]

I suggest to add the key word "const" to the type specifiers for parameters like "filename" (functions "bsmap" and "dx_build").
Would you like to apply the advices from an article to more places in your source files?

[rchiossi]

Those are welcome improvements. Can you sendo me a pull request with them?

[elfring]

I find that the status should be clarified a bit more for the member "filename" of the data structure "ByteStream". Would you like to store a modifiable string there?

[rchiossi]

Not really. The filename should be set only once for each bytestream object.