Refactor fmt_human() and fix a bug

It is not necessary to copy memory or anything. Just keep a pointer to
the active prefix-array and assign the length of the arrays to a
variable.

Make the code more readable by using a switch, be more strict when an
invalid base is passed to it and fix a small oversight in the bottom of
the code where the base 1024 was forgotten to generalized.

util.h

@@ -10,5 +10,5 @@ void die(const char *, ...);
 
 int esnprintf(char *str, size_t size, const char *fmt, ...);
 const char *bprintf(const char *fmt, ...);
-const char *fmt_human(size_t num, int iec);
+const char *fmt_human(size_t num, int base);
 int pscanf(const char *path, const char *fmt, ...);