[barcode] unsigned and signed char

[Alessandro Rubini]

since compilation spits a zillion warning on a semi-recent distro,
I tried to move all "unsigned char" to "char". OTherwise any
strlen() strchr() or similare function will spit a warning.

Are there any compelling reasong to stick with unsigned char
instead of letting the compile choose the correct signedness?

Since they are not used as numbers, I don't think we have problems.
I tend to use unsigned char as most of my data is byte, so that
may be the reason, but I don't remember.

If I don't hear opinions against it, I'll use the patch.

/alessandro