a code reduction function addition to cam_xpt

[mjacob at freebsd.org]

> Would not be better a preprocessing stub?
>
> something like:
>
> #define XPT_PRINT(path, fmt, ...) do {
>           \
>             xpt_print_path(path);
>                     \
>             printf(fmt, __VA_ARGS__);
>                 \
> } while (0)
>

Well, it's uglier (IMO0, doesn't get formal checking (which I forgot to 
add to the define in cam_xpt.h), and you can't port it to systems that 
have an older compiler. Other than that, it's probably better.