uprf: We don't have to implement it ourselves but can use strncasecmp()

uprf: We don't have to implement it ourselves but can use strncasecmp()
Also clarified what an ``unsigned prefix'' really is.
I haven't changed all uses of uprf() in the code to strncasecmp(). This
could be done. I'm not sure if it would enhance the clarity of the code.
We don't need a prototype for strncasecmp(), because it's declared in
strings.h.