Implement fmt_human_2() and fmt_human_10()

These functions take the raw number and a unit and automatically
print it out "scaled down" to a proper SI-prefix, for powers of 2
and 10 respectively.

Apply them to the 2-power cases and keep the 10-power for a later
commit.

util.h

@@ -10,5 +10,6 @@ void die(const char *, ...);
 
 int esnprintf(char *str, size_t size, const char *fmt, ...);
 const char *bprintf(const char *fmt, ...);
-const char *fmt_scaled(size_t);
+const char *fmt_human_2(size_t num, char *unit);
+const char *fmt_human_10(size_t num, char *unit);
 int pscanf(const char *path, const char *fmt, ...);